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I'm using JSLint to go through JavaScript, and it's returning many suggestions to replace == (two equals signs) with === (three equals signs) when doing things like comparing idSele_UNVEHtype.value.length == 0 inside of an if statement.
Is there a performance benefit to replacing == with ===?
Any performance improvement would be welcomed as many comparison operators exist.
If no type conversion takes place, would there be a performance gain over ==?
The strict equality operator (===) behaves identically to the abstract equality operator (==) except no type conversion is done, and the types must be the same to be considered equal.
Reference: Javascript Tutorial: Comparison Operators
The == operator will compare for equality after doing any necessary type conversions. The === operator will not do the conversion, so if two values are not the same type === will simply return false. Both are equally quick.
To quote Douglas Crockford's excellent JavaScript: The Good Parts,
JavaScript has two sets of equality operators: === and !==, and their evil twins == and !=. The good ones work the way you would expect. If the two operands are of the same type and have the same value, then === produces true and !== produces false. The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. the rules by which they do that are complicated and unmemorable. These are some of the interesting cases:
'' == '0' // false
0 == '' // true
0 == '0' // true
false == 'false' // false
false == '0' // true
false == undefined // false
false == null // false
null == undefined // true
' \t\r\n ' == 0 // true
The lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use === and !==. All of the comparisons just shown produce false with the === operator.
Update:
A good point was brought up by #Casebash in the comments and in #Phillipe Laybaert's answer concerning objects. For objects, == and === act consistently with one another (except in a special case).
var a = [1,2,3];
var b = [1,2,3];
var c = { x: 1, y: 2 };
var d = { x: 1, y: 2 };
var e = "text";
var f = "te" + "xt";
a == b // false
a === b // false
c == d // false
c === d // false
e == f // true
e === f // true
The special case is when you compare a primitive with an object that evaluates to the same primitive, due to its toString or valueOf method. For example, consider the comparison of a string primitive with a string object created using the String constructor.
"abc" == new String("abc") // true
"abc" === new String("abc") // false
Here the == operator is checking the values of the two objects and returning true, but the === is seeing that they're not the same type and returning false. Which one is correct? That really depends on what you're trying to compare. My advice is to bypass the question entirely and just don't use the String constructor to create string objects from string literals.
Reference
http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3
Using the == operator (Equality)
true == 1; //true, because 'true' is converted to 1 and then compared
"2" == 2; //true, because "2" is converted to 2 and then compared
Using the === operator (Identity)
true === 1; //false
"2" === 2; //false
This is because the equality operator == does type coercion, meaning that the interpreter implicitly tries to convert the values before comparing.
On the other hand, the identity operator === does not do type coercion, and thus does not convert the values when comparing.
Here's an interesting visualisation of the equality comparison between == and ===.
Source: https://github.com/dorey/JavaScript-Equality-Table (demo, unified demo)
var1 === var2
When using === for JavaScript equality testing, everything is as is.
Nothing gets converted before being evaluated.
var1 == var2
When using == for JavaScript equality testing, some funky conversions take place.
Summary of equality in Javascript
Conclusion:
Always use ===, unless you fully understand the funky conversions that take place with ==.
In the answers here, I didn't read anything about what equal means. Some will say that === means equal and of the same type, but that's not really true. It actually means that both operands reference the same object, or in case of value types, have the same value.
So, let's take the following code:
var a = [1,2,3];
var b = [1,2,3];
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
The same here:
var a = { x: 1, y: 2 };
var b = { x: 1, y: 2 };
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
Or even:
var a = { };
var b = { };
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
This behavior is not always obvious. There's more to the story than being equal and being of the same type.
The rule is:
For value types (numbers):
a === b returns true if a and b have the same value and are of the same type
For reference types:
a === b returns true if a and b reference the exact same object
For strings:
a === b returns true if a and b are both strings and contain the exact same characters
Strings: the special case...
Strings are not value types, but in Javascript they behave like value types, so they will be "equal" when the characters in the string are the same and when they are of the same length (as explained in the third rule)
Now it becomes interesting:
var a = "12" + "3";
var b = "123";
alert(a === b); // returns true, because strings behave like value types
But how about this?:
var a = new String("123");
var b = "123";
alert(a === b); // returns false !! (but they are equal and of the same type)
I thought strings behave like value types? Well, it depends who you ask... In this case a and b are not the same type. a is of type Object, while b is of type string. Just remember that creating a string object using the String constructor creates something of type Object that behaves as a string most of the time.
Let me add this counsel:
If in doubt, read the specification!
ECMA-262 is the specification for a scripting language of which JavaScript is a dialect. Of course in practice it matters more how the most important browsers behave than an esoteric definition of how something is supposed to be handled. But it is helpful to understand why new String("a") !== "a".
Please let me explain how to read the specification to clarify this question. I see that in this very old topic nobody had an answer for the very strange effect. So, if you can read a specification, this will help you in your profession tremendously. It is an acquired skill. So, let's continue.
Searching the PDF file for === brings me to page 56 of the specification: 11.9.4. The Strict Equals Operator ( === ), and after wading through the specificationalese I find:
11.9.6 The Strict Equality Comparison Algorithm
The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:
1. If Type(x) is different from Type(y), return false.
2. If Type(x) is Undefined, return true.
3. If Type(x) is Null, return true.
4. If Type(x) is not Number, go to step 11.
5. If x is NaN, return false.
6. If y is NaN, return false.
7. If x is the same number value as y, return true.
8. If x is +0 and y is −0, return true.
9. If x is −0 and y is +0, return true.
10. Return false.
11. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
12. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
13. Return true if x and y refer to the same object or if they refer to objects joined to each other (see 13.1.2). Otherwise, return false.
Interesting is step 11. Yes, strings are treated as value types. But this does not explain why new String("a") !== "a". Do we have a browser not conforming to ECMA-262?
Not so fast!
Let's check the types of the operands. Try it out for yourself by wrapping them in typeof(). I find that new String("a") is an object, and step 1 is used: return false if the types are different.
If you wonder why new String("a") does not return a string, how about some exercise reading a specification? Have fun!
Aidiakapi wrote this in a comment below:
From the specification
11.2.2 The new Operator:
If Type(constructor) is not Object, throw a TypeError exception.
With other words, if String wouldn't be of type Object it couldn't be used with the new operator.
new always returns an Object, even for String constructors, too. And alas! The value semantics for strings (see step 11) is lost.
And this finally means: new String("a") !== "a".
I tested this in Firefox with Firebug using code like this:
console.time("testEquality");
var n = 0;
while (true) {
n++;
if (n == 100000)
break;
}
console.timeEnd("testEquality");
and
console.time("testTypeEquality");
var n = 0;
while (true) {
n++;
if (n === 100000)
break;
}
console.timeEnd("testTypeEquality");
My results (tested five times each and averaged):
==: 115.2
===: 114.4
So I'd say that the miniscule difference (this is over 100000 iterations, remember) is negligible. Performance isn't a reason to do ===. Type safety (well, as safe as you're going to get in JavaScript), and code quality is.
In PHP and JavaScript, it is a strict equality operator. Which means, it will compare both type and values.
In JavaScript it means of the same value and type.
For example,
4 == "4" // will return true
but
4 === "4" // will return false
Why == is so unpredictable?
What do you get when you compare an empty string "" with the number zero 0?
true
Yep, that's right according to == an empty string and the number zero are the same time.
And it doesn't end there, here's another one:
'0' == false // true
Things get really weird with arrays.
[1] == true // true
[] == false // true
[[]] == false // true
[0] == false // true
Then weirder with strings
[1,2,3] == '1,2,3' // true - REALLY?!
'\r\n\t' == 0 // true - Come on!
It get's worse:
When is equal not equal?
let A = '' // empty string
let B = 0 // zero
let C = '0' // zero string
A == B // true - ok...
B == C // true - so far so good...
A == C // **FALSE** - Plot twist!
Let me say that again:
(A == B) && (B == C) // true
(A == C) // **FALSE**
And this is just the crazy stuff you get with primitives.
It's a whole new level of crazy when you use == with objects.
At this point your probably wondering...
Why does this happen?
Well it's because unlike "triple equals" (===) which just checks if two values are the same.
== does a whole bunch of other stuff.
It has special handling for functions, special handling for nulls, undefined, strings, you name it.
It get's pretty wacky.
In fact, if you tried to write a function that does what == does it would look something like this:
function isEqual(x, y) { // if `==` were a function
if(typeof y === typeof x) return y === x;
// treat null and undefined the same
var xIsNothing = (y === undefined) || (y === null);
var yIsNothing = (x === undefined) || (x === null);
if(xIsNothing || yIsNothing) return (xIsNothing && yIsNothing);
if(typeof y === "function" || typeof x === "function") {
// if either value is a string
// convert the function into a string and compare
if(typeof x === "string") {
return x === y.toString();
} else if(typeof y === "string") {
return x.toString() === y;
}
return false;
}
if(typeof x === "object") x = toPrimitive(x);
if(typeof y === "object") y = toPrimitive(y);
if(typeof y === typeof x) return y === x;
// convert x and y into numbers if they are not already use the "+" trick
if(typeof x !== "number") x = +x;
if(typeof y !== "number") y = +y;
// actually the real `==` is even more complicated than this, especially in ES6
return x === y;
}
function toPrimitive(obj) {
var value = obj.valueOf();
if(obj !== value) return value;
return obj.toString();
}
So what does this mean?
It means == is complicated.
Because it's complicated it's hard to know what's going to happen when you use it.
Which means you could end up with bugs.
So the moral of the story is...
Make your life less complicated.
Use === instead of ==.
The End.
The === operator is called a strict comparison operator, it does differ from the == operator.
Lets take 2 vars a and b.
For "a == b" to evaluate to true a and b need to be the same value.
In the case of "a === b" a and b must be the same value and also the same type for it to evaluate to true.
Take the following example
var a = 1;
var b = "1";
if (a == b) //evaluates to true as a and b are both 1
{
alert("a == b");
}
if (a === b) //evaluates to false as a is not the same type as b
{
alert("a === b");
}
In summary; using the == operator might evaluate to true in situations where you do not want it to so using the === operator would be safer.
In the 90% usage scenario it won't matter which one you use, but it is handy to know the difference when you get some unexpected behaviour one day.
=== checks same sides are equal in type as well as value.
Example:
'1' === 1 // will return "false" because `string` is not a `number`
Common example:
0 == '' // will be "true", but it's very common to want this check to be "false"
Another common example:
null == undefined // returns "true", but in most cases a distinction is necessary
Many times an untyped check would be handy because you do not care if the value is either undefined, null, 0 or ""
Javascript execution flow diagram for strict equality / Comparison '==='
Javascript execution flow diagram for non strict equality / comparison '=='
JavaScript === vs == .
0==false // true
0===false // false, because they are of a different type
1=="1" // true, auto type coercion
1==="1" // false, because they are of a different type
It means equality without type coercion
type coercion means JavaScript do not automatically convert any other data types to string data types
0==false // true,although they are different types
0===false // false,as they are different types
2=='2' //true,different types,one is string and another is integer but
javaScript convert 2 to string by using == operator
2==='2' //false because by using === operator ,javaScript do not convert
integer to string
2===2 //true because both have same value and same types
In a typical script there will be no performance difference. More important may be the fact that thousand "===" is 1 KB heavier than thousand "==" :) JavaScript profilers can tell you if there is a performance difference in your case.
But personally I would do what JSLint suggests. This recommendation is there not because of performance issues, but because type coercion means ('\t\r\n' == 0) is true.
The equal comparison operator == is confusing and should be avoided.
If you HAVE TO live with it, then remember the following 3 things:
It is not transitive: (a == b) and (b == c) does not lead to (a == c)
It's mutually exclusive to its negation: (a == b) and (a != b) always hold opposite Boolean values, with all a and b.
In case of doubt, learn by heart the following truth table:
EQUAL OPERATOR TRUTH TABLE IN JAVASCRIPT
Each row in the table is a set of 3 mutually "equal" values, meaning that any 2 values among them are equal using the equal == sign*
** STRANGE: note that any two values on the first column are not equal in that sense.**
'' == 0 == false // Any two values among these 3 ones are equal with the == operator
'0' == 0 == false // Also a set of 3 equal values, note that only 0 and false are repeated
'\t' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\r' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\n' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\t\r\n' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
null == undefined // These two "default" values are not-equal to any of the listed values above
NaN // NaN is not equal to any thing, even to itself.
There is unlikely to be any performance difference between the two operations in your usage. There is no type-conversion to be done because both parameters are already the same type. Both operations will have a type comparison followed by a value comparison.
Simply
== means comparison between operands with type coercion
and
=== means comparison between operands without type coercion.
Type coercion in JavaScript means automatically converting data types to other data types.
For example:
123 == "123" // Returns true, because JS coerces string "123" to number 123
// and then goes on to compare `123 == 123`.
123 === "123" // Returns false, because JS does not coerce values of different types here.
Yes! It does matter.
=== operator in javascript checks value as well as type where as == operator just checks the value (does type conversion if required).
You can easily test it. Paste following code in an HTML file and open it in browser
<script>
function onPageLoad()
{
var x = "5";
var y = 5;
alert(x === 5);
};
</script>
</head>
<body onload='onPageLoad();'>
You will get 'false' in alert. Now modify the onPageLoad() method to alert(x == 5); you will get true.
As a rule of thumb, I would generally use === instead of == (and !== instead of !=).
Reasons are explained in in the answers above and also Douglas Crockford is pretty clear about it (JavaScript: The Good Parts).
However there is one single exception:
== null is an efficient way to check for 'is null or undefined':
if( value == null ){
// value is either null or undefined
}
For example jQuery 1.9.1 uses this pattern 43 times, and the JSHint syntax checker even provides the eqnull relaxing option for this reason.
From the jQuery style guide:
Strict equality checks (===) should be used in favor of ==. The only
exception is when checking for undefined and null by way of null.
// Check for both undefined and null values, for some important reason.
undefOrNull == null;
EDIT 2021-03:
Nowadays most browsers
support the Nullish coalescing operator (??)
and the Logical nullish assignment (??=), which allows a more concise way to
assign a default value if a variable is null or undefined, for example:
if (a.speed == null) {
// Set default if null or undefined
a.speed = 42;
}
can be written as any of these forms
a.speed ??= 42;
a.speed ?? a.speed = 42;
a.speed = a.speed ?? 42;
It's a strict check test.
It's a good thing especially if you're checking between 0 and false and null.
For example, if you have:
$a = 0;
Then:
$a==0;
$a==NULL;
$a==false;
All returns true and you may not want this. Let's suppose you have a function that can return the 0th index of an array or false on failure. If you check with "==" false, you can get a confusing result.
So with the same thing as above, but a strict test:
$a = 0;
$a===0; // returns true
$a===NULL; // returns false
$a===false; // returns false
=== operator checks the values as well as the types of the variables for equality.
== operator just checks the value of the variables for equality.
JSLint sometimes gives you unrealistic reasons to modify stuff. === has exactly the same performance as == if the types are already the same.
It is faster only when the types are not the same, in which case it does not try to convert types but directly returns a false.
So, IMHO, JSLint maybe used to write new code, but useless over-optimizing should be avoided at all costs.
Meaning, there is no reason to change == to === in a check like if (a == 'test') when you know it for a fact that a can only be a String.
Modifying a lot of code that way wastes developers' and reviewers' time and achieves nothing.
A simple example is
2 == '2' -> true, values are SAME because of type conversion.
2 === '2' -> false, values are NOT SAME because of no type conversion.
The top 2 answers both mentioned == means equality and === means identity. Unfortunately, this statement is incorrect.
If both operands of == are objects, then they are compared to see if they are the same object. If both operands point to the same object, then the equal operator returns true. Otherwise,
the two are not equal.
var a = [1, 2, 3];
var b = [1, 2, 3];
console.log(a == b) // false
console.log(a === b) // false
In the code above, both == and === get false because a and b are not the same objects.
That's to say: if both operands of == are objects, == behaves same as ===, which also means identity. The essential difference of this two operators is about type conversion. == has conversion before it checks equality, but === does not.
The problem is that you might easily get into trouble since JavaScript have a lot of implicit conversions meaning...
var x = 0;
var isTrue = x == null;
var isFalse = x === null;
Which pretty soon becomes a problem. The best sample of why implicit conversion is "evil" can be taken from this code in MFC / C++ which actually will compile due to an implicit conversion from CString to HANDLE which is a pointer typedef type...
CString x;
delete x;
Which obviously during runtime does very undefined things...
Google for implicit conversions in C++ and STL to get some of the arguments against it...
From the core javascript reference
=== Returns true if the operands are strictly equal (see above)
with no type conversion.
Equality comparison:
Operator ==
Returns true, when both operands are equal. The operands are converted to the same type before being compared.
>>> 1 == 1
true
>>> 1 == 2
false
>>> 1 == '1'
true
Equality and type comparison:
Operator ===
Returns true if both operands are equal and of the same type. It's generally
better and safer if you compare this way, because there's no behind-the-scenes type conversions.
>>> 1 === '1'
false
>>> 1 === 1
true
Here is a handy comparison table that shows the conversions that happen and the differences between == and ===.
As the conclusion states:
"Use three equals unless you fully understand the conversions that take
place for two-equals."
http://dorey.github.io/JavaScript-Equality-Table/
null and undefined are nothingness, that is,
var a;
var b = null;
Here a and b do not have values. Whereas, 0, false and '' are all values. One thing common beween all these are that they are all falsy values, which means they all satisfy falsy conditions.
So, the 0, false and '' together form a sub-group. And on other hand, null & undefined form the second sub-group. Check the comparisons in the below image. null and undefined would equal. The other three would equal to each other. But, they all are treated as falsy conditions in JavaScript.
This is same as any object (like {}, arrays, etc.), non-empty string & Boolean true are all truthy conditions. But, they are all not equal.
Related to Is there a "null coalescing" operator in JavaScript? - JavaScript now has a ?? operator which I see is in use more frequently. Previously most JavaScript code used ||.
let userAge = null
// These values will be the same.
let age1 = userAge || 21
let age2 = userAge ?? 21
In what circumstances will ?? and || behave differently?
The OR operator || uses the right value if left is falsy, while the nullish coalescing operator ?? uses the right value if left is null or undefined.
These operators are often used to provide a default value if the first one is missing.
But the OR operator || can be problematic if your left value might contain "" or 0 or false (because these are falsy values):
console.log(12 || "not found") // 12
console.log(0 || "not found") // "not found"
console.log("jane" || "not found") // "jane"
console.log("" || "not found") // "not found"
console.log(true || "not found") // true
console.log(false || "not found") // "not found"
console.log(undefined || "not found") // "not found"
console.log(null || "not found") // "not found"
In many cases, you might only want the right value if left is null or undefined. That's what the nullish coalescing operator ?? is for:
console.log(12 ?? "not found") // 12
console.log(0 ?? "not found") // 0
console.log("jane" ?? "not found") // "jane"
console.log("" ?? "not found") // ""
console.log(true ?? "not found") // true
console.log(false ?? "not found") // false
console.log(undefined ?? "not found") // "not found"
console.log(null ?? "not found") // "not found"
While the ?? operator isn't available in current LTS versions of Node (v10 and v12), you can use it with some versions of TypeScript or Node:
The ?? operator was added to TypeScript 3.7 back in November 2019.
And more recently, the ?? operator was included in ES2020, which is supported by Node 14 (released in April 2020).
When the nullish coalescing operator ?? is supported, I typically use it instead of the OR operator || (unless there's a good reason not to).
In short
The Nullish Coalescing Operator ?? distinguishes between:
nullish values (null, undefined)
falsey but defined values (false, 0, '' etc.)
|| (logical OR) treats both of these the same.
I created a simple graphic to illustrate the relationship of nullish and falsey values in JavaScript:
Further explanation:
let x, y
x = 0
y = x || 'default' // y = 'default'
y = x ?? 'default' // y = 0
As seen above, the difference between the operators ?? and || is that one is checking for nullish values and one is checking for falsey values. However, there are many instances where they behave the same. That is because in JavaScript every nullish value is also falsey (but not every falsey value is nullish).
Using what we learned above we can create a few examples for different behavior:
let y
y = false || 'default' // y = 'default'
y = false ?? 'default' // y = false
y = 0n || 'default' // y = 'default'
y = 0n ?? 'default' // y = 0n
y = NaN || 'default' // y = 'default'
y = NaN ?? 'default' // y = NaN
y = '' || 'default' // y = 'default'
y = '' ?? 'default' // y = ''
Since the new Nullish Coalescing Operator can differentiate between no value and a falsey value, it can be beneficial if you, for example, need to check if there is no String or an empty String. Generally, you probably want to use ?? instead of || most of the time.
Last and also least here are the two instances where they behave the same:
let y
y = null || 'default' // y = 'default'
y = null ?? 'default' // y = 'default'
y = undefined || 'default' // y = 'default'
y = undefined ?? 'default' // y = 'default'
As a very short rule, you could look at it the opposite way:
|| (or) returns the first "truthy" value (or the last value if no "truthy" value exists)
?? (nullish coalescing) returns the first "defined" value (or the last value if no "defined" value exists)
Example
x = false || true; // --> true (the first 'truthy' value - parameter 2)
x = false ?? true; // --> false (the first 'defined' value - parameter 1)
As described in the MDN:
Contrary to the logical OR (||) operator, the left operand is returned if it is a falsy value which is not null or undefined. In other words, if you use || to provide some default value to another variable foo, you may encounter unexpected behaviors if you consider some falsy values as usable (eg. '' or 0). See below for more examples.
And also in the answer to the question you linked:
Regardless of the type of the first operand, if casting it to a Boolean results in false, the assignment will use the second operand. Beware of all the cases below:
alert(Boolean(null)); // false
alert(Boolean(undefined)); // false
alert(Boolean(0)); // false
alert(Boolean("")); // false
alert(Boolean("false")); // true -- gotcha! :)
Base on MDN docs:
Contrary to the logical OR (||) operator, the left operand is returned if it is a falsy value which is not null or undefined.
Conceptual Example:
When use || for a falsy value that is NOT undefined or null:
false || 'name'; // ==> the 'name' is returned
But when using ?? for above case:
false ?? 'name'; // ==> the false is returned
Real Example:
Assume, we have a phone variable that is not mandatory in our form and the empty string ('') is valid for us and we wanna doSomething() if the phone variable is null or undefined, now guess what:
When use || for a falsy value that is NOT undefined or null:
const phone = ''; // assume it became empty string from some action
phone || doSomething(); // ==> damn, the doSomething is run
// but we want to run it if it's `undefined` or `null` the empty string is valid for us
But when using ?? for above case:
const phone = ''; // same assumption like above
phone ?? doSomething(); // ==> yeah, that's right
// the empty string is valid for us and the doSomething is not run
Note: actually it is an example and in a real project you can have a better sense of this lovely operation usage.
Attention: for the undefined or null both of them act like each other.
function f(input) {
const val = input || 1;
return 41 + val;
}
function g(input) {
const val = input ?? 1;
return 41 + val;
}
console.log("using ||:", f(0));
console.log("using ??:", g(0));
The null(ish) coalescing operator only works with null and undefined. So, use the null(ish) coalescing operator when you don't want those values but you will otherwise accept other falsy values:
console.log(null ?? "nullish");
console.log(undefined ?? "nullish");
console.log("" ?? "nullish");
console.log(0 ?? "nullish");
console.log(false ?? "nullish");
The logical OR will skip over any falsy value and give you the other thing.
The logical OR will skip over any falsy value and give you the other parameter. This does work and is by now idiomatic, however it is not always what you want:
console.log(null || "falsy");
console.log(undefined || "falsy");
console.log("" || "falsy");
console.log(0 || "falsy");
console.log(false || "falsy");
Here are few rules for how to determine which one you need. The simplest tests:
do you only want to protect against null (and undefined - it's usually the same thing)? Then use ??. If you aren't sure, it's probably a good idea to default to the nullish coalescing operator.
do you know you also don't want 0 or "", for example? Then use ||.
The second one is where it actually gets trickly. How do you know you need to discard falsy values? Well, the very first snippet shows what happens if you do that: f(0) will discard the zero and thus produce a different result. This is a somewhat common source of bugs. Since the construct a = b || c is common to introduce a fallback value (in this case c) it might accidentally fall back to it when you didn't mean to.
function updateAge(years) {
var yearsToAdd = years || 1;
return this.age + yearsToAdd
}
This works if you want to provide a fallback for calling updateAge() (no argument) but fails if you call updateAge(0) (no update). Similarly, you could have:
function sayMyName(greeting) {
var prefix = greeting || "Hello, my name is ";
return prefix + this.firstName;
}
Again, this works for sayMyName() (no argument) but fails for sayMyName("") (explicitly no greeting).
To generalise, if you are providing a fallback value that's different to the falsy value, then you might have a problem. However, if your fallback is the falsy value - num || 0 or str || "" then it doesn't really matter.
It's rare (or should be) that you might expect a mixed type (number, or string, or object, etc) and provide a fallback to it: input || fallback is valid but will reject empty strings and zeroes. It's usually going to be wrong unless you explicitly don't want any of those.
To put it simply, you should likely use the nullish coalescing operator ?? at all times. It will be less cognitive load to figure out whether it's a potential bug or not. There is also not much reason to avoid it. It's newer than the boolean OR, so it doesn't work on older environments, that's true, however nowadays you should likely be transpiling your code for those. If you cannot or prefer not to, then you don't have a choice and should use the boolean OR.
When the || is being used as a boolean conditional that goes to false. For example:
let userAge = false
// These values will be the same.
let age1 = userAge || 21 // 21
let age2 = userAge ?? 21 // false
The logical OR will still give the next value for anything that doesn't evaluate to true. So it gives the value 21 in this case. Where ?? only handles null and undefined.
In short, when you care that let a variable assigned from a possible null undefined source, and you wish to provide a default value to the variable, use the nullish coalescing operator ??.
If you want to avoid messing yourself around with the JavaScript definition of falsy truthy [1], then don't use ||.
nullish coalescing operator ??
JavaScript definition of falsy
a real example in angular
/* parse the url pattern
/search?keyword=mykeyword&page=3
/search?keyword=mykeyword
*/
ngOnInit(): void {
const keyword = this.route.snapshot.queryParamMap.get('keyword') ?? 'default keyword';
const page = this.route.snapshot.queryParamMap.get('page') ?? '0';
this.queryResult$ = this.service.getQueryResult(keyword, +page);
this.keyword = keyword;
}
[1]:
Each programming language has its own definition of falsy truthy. The basic approach to tell whether a value is a truthy value is that if the explicit type conversion function bool(value) results in true, then the value is truthy.
For those might work across languages, PHP and C# also have ??. See PHP, C#.
var connections = {key: 1234}
var thisconnection = connections["key"] || {status:"new"};`
In the above code, the variable thisconnection is subjected to the Logical operator '||'
When I simply type the following code in the console, thisconnection takes the value of the connections["key"] in any case.
Why don't you try it?
var connections = {key: 1234};
var thisconnection = connections["key"] || {status:"new"};
console.log(thisconnection); // returns 1234
Explanation:
If connections.key returned 0, NaN, false, undefined, "" (empty string), or any value that evaluated to false, it would continue evaluating the next || expression.
However, if the expression evaluates to a truthy value, the whole statement is finished with evaluation. This is known as short-circuiting:
As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:
false && (anything) is short-circuit evaluated to false.
true || (anything) is short-circuit evaluated to true.
Since 1234 is a truthy value, evaluation stops and the value 1234 is assigned.
In the following statement
var thisconnection = connections["key"] || {status:"new"};
You are saying if connection["key"] exists assign connection["key"] to thisconnection, other wise assign {status:new} to thisconnection.
It's same as following code
if(connections["key"]){
thisconnection = connection["key"]
}
else{
thisconnection = {status:new}
}
if connections["key"] is nil then {status:"new"} will be the value of variable
In JavaScript, || and && operators have a specific behavior when used in variable assignment. If you have something like this:
var theResult = a || b || c || d;
Then, the variable theResult won't be a boolean as we expect in other languages.
If a is truhty, then theResult = a. Else, if b is truhty, then theResult = b. And so on... If all variables are falsy, then theResult = d because d is the last statement in the or condition.
The rule is the following:
the value of theResult will be the value of the variable in a || b || c || d which break the chain
So, since we have only OR conditions, then the first value which resolve to true will break the chain. I mean, if a === true, then it's not needed to evaluate the rest of the condition because true || false || whatEver || weDontCare will always be true. If none of the variable breaks the chain, then the last variable is assigned to theResult.
First it will check connections["key"],
If it is not null, then the value of thisconnection with contain value of connections["key"]. Otherwise thisconnection will contain value of {status:"new"}.
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I'm using JSLint to go through JavaScript, and it's returning many suggestions to replace == (two equals signs) with === (three equals signs) when doing things like comparing idSele_UNVEHtype.value.length == 0 inside of an if statement.
Is there a performance benefit to replacing == with ===?
Any performance improvement would be welcomed as many comparison operators exist.
If no type conversion takes place, would there be a performance gain over ==?
The strict equality operator (===) behaves identically to the abstract equality operator (==) except no type conversion is done, and the types must be the same to be considered equal.
Reference: JavaScript Tutorial: Comparison Operators
The == operator will compare for equality after doing any necessary type conversions. The === operator will not do the conversion, so if two values are not the same type === will simply return false. Both are equally quick.
To quote Douglas Crockford's excellent JavaScript: The Good Parts,
JavaScript has two sets of equality operators: === and !==, and their evil twins == and !=. The good ones work the way you would expect. If the two operands are of the same type and have the same value, then === produces true and !== produces false. The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. The rules by which they do that are complicated and unmemorable. These are some of the interesting cases:
'' == '0' // false
0 == '' // true
0 == '0' // true
false == 'false' // false
false == '0' // true
false == undefined // false
false == null // false
null == undefined // true
' \t\r\n ' == 0 // true
The lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use === and !==. All of the comparisons just shown produce false with the === operator.
Update
A good point was brought up by #Casebash in the comments and in #Phillipe Laybaert's answer concerning objects. For objects, == and === act consistently with one another (except in a special case).
var a = [1,2,3];
var b = [1,2,3];
var c = { x: 1, y: 2 };
var d = { x: 1, y: 2 };
var e = "text";
var f = "te" + "xt";
a == b // false
a === b // false
c == d // false
c === d // false
e == f // true
e === f // true
The special case is when you compare a primitive with an object that evaluates to the same primitive, due to its toString or valueOf method. For example, consider the comparison of a string primitive with a string object created using the String constructor.
"abc" == new String("abc") // true
"abc" === new String("abc") // false
Here the == operator is checking the values of the two objects and returning true, but the === is seeing that they're not the same type and returning false. Which one is correct? That really depends on what you're trying to compare. My advice is to bypass the question entirely and just don't use the String constructor to create string objects from string literals.
Reference
https://262.ecma-international.org/5.1/#sec-11.9.3
Using the == operator (Equality)
true == 1; //true, because 'true' is converted to 1 and then compared
"2" == 2; //true, because "2" is converted to 2 and then compared
Using the === operator (Identity)
true === 1; //false
"2" === 2; //false
This is because the equality operator == does type coercion, meaning that the interpreter implicitly tries to convert the values before comparing.
On the other hand, the identity operator === does not do type coercion, and thus does not convert the values when comparing.
Here's an interesting visualisation of the equality comparison between == and ===.
Source: https://github.com/dorey/JavaScript-Equality-Table (demo, unified demo)
var1 === var2
When using === for JavaScript equality testing, everything is as is.
Nothing gets converted before being evaluated.
var1 == var2
When using == for JavaScript equality testing, some funky conversions take place.
Summary of equality in Javascript
Conclusion:
Always use ===, unless you fully understand the funky conversions that take place with ==.
In the answers here, I didn't read anything about what equal means. Some will say that === means equal and of the same type, but that's not really true. It actually means that both operands reference the same object, or in case of value types, have the same value.
So, let's take the following code:
var a = [1,2,3];
var b = [1,2,3];
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
The same here:
var a = { x: 1, y: 2 };
var b = { x: 1, y: 2 };
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
Or even:
var a = { };
var b = { };
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
This behavior is not always obvious. There's more to the story than being equal and being of the same type.
The rule is:
For value types (numbers):
a === b returns true if a and b have the same value and are of the same type
For reference types:
a === b returns true if a and b reference the exact same object
For strings:
a === b returns true if a and b are both strings and contain the exact same characters
Strings: the special case...
Strings are not value types, but in Javascript they behave like value types, so they will be "equal" when the characters in the string are the same and when they are of the same length (as explained in the third rule)
Now it becomes interesting:
var a = "12" + "3";
var b = "123";
alert(a === b); // returns true, because strings behave like value types
But how about this?:
var a = new String("123");
var b = "123";
alert(a === b); // returns false !! (but they are equal and of the same type)
I thought strings behave like value types? Well, it depends who you ask... In this case a and b are not the same type. a is of type Object, while b is of type string. Just remember that creating a string object using the String constructor creates something of type Object that behaves as a string most of the time.
Let me add this counsel:
If in doubt, read the specification!
ECMA-262 is the specification for a scripting language of which JavaScript is a dialect. Of course in practice it matters more how the most important browsers behave than an esoteric definition of how something is supposed to be handled. But it is helpful to understand why new String("a") !== "a".
Please let me explain how to read the specification to clarify this question. I see that in this very old topic nobody had an answer for the very strange effect. So, if you can read a specification, this will help you in your profession tremendously. It is an acquired skill. So, let's continue.
Searching the PDF file for === brings me to page 56 of the specification: 11.9.4. The Strict Equals Operator ( === ), and after wading through the specificationalese I find:
11.9.6 The Strict Equality Comparison Algorithm
The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:
1. If Type(x) is different from Type(y), return false.
2. If Type(x) is Undefined, return true.
3. If Type(x) is Null, return true.
4. If Type(x) is not Number, go to step 11.
5. If x is NaN, return false.
6. If y is NaN, return false.
7. If x is the same number value as y, return true.
8. If x is +0 and y is −0, return true.
9. If x is −0 and y is +0, return true.
10. Return false.
11. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
12. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
13. Return true if x and y refer to the same object or if they refer to objects joined to each other (see 13.1.2). Otherwise, return false.
Interesting is step 11. Yes, strings are treated as value types. But this does not explain why new String("a") !== "a". Do we have a browser not conforming to ECMA-262?
Not so fast!
Let's check the types of the operands. Try it out for yourself by wrapping them in typeof(). I find that new String("a") is an object, and step 1 is used: return false if the types are different.
If you wonder why new String("a") does not return a string, how about some exercise reading a specification? Have fun!
Aidiakapi wrote this in a comment below:
From the specification
11.2.2 The new Operator:
If Type(constructor) is not Object, throw a TypeError exception.
With other words, if String wouldn't be of type Object it couldn't be used with the new operator.
new always returns an Object, even for String constructors, too. And alas! The value semantics for strings (see step 11) is lost.
And this finally means: new String("a") !== "a".
I tested this in Firefox with Firebug using code like this:
console.time("testEquality");
var n = 0;
while (true) {
n++;
if (n == 100000)
break;
}
console.timeEnd("testEquality");
and
console.time("testTypeEquality");
var n = 0;
while (true) {
n++;
if (n === 100000)
break;
}
console.timeEnd("testTypeEquality");
My results (tested five times each and averaged):
==: 115.2
===: 114.4
So I'd say that the miniscule difference (this is over 100000 iterations, remember) is negligible. Performance isn't a reason to do ===. Type safety (well, as safe as you're going to get in JavaScript), and code quality is.
In PHP and JavaScript, it is a strict equality operator. Which means, it will compare both type and values.
In JavaScript it means of the same value and type.
For example,
4 == "4" // will return true
but
4 === "4" // will return false
Why == is so unpredictable?
What do you get when you compare an empty string "" with the number zero 0?
true
Yep, that's right according to == an empty string and the number zero are the same time.
And it doesn't end there, here's another one:
'0' == false // true
Things get really weird with arrays.
[1] == true // true
[] == false // true
[[]] == false // true
[0] == false // true
Then weirder with strings
[1,2,3] == '1,2,3' // true - REALLY?!
'\r\n\t' == 0 // true - Come on!
It get's worse:
When is equal not equal?
let A = '' // empty string
let B = 0 // zero
let C = '0' // zero string
A == B // true - ok...
B == C // true - so far so good...
A == C // **FALSE** - Plot twist!
Let me say that again:
(A == B) && (B == C) // true
(A == C) // **FALSE**
And this is just the crazy stuff you get with primitives.
It's a whole new level of crazy when you use == with objects.
At this point your probably wondering...
Why does this happen?
Well it's because unlike "triple equals" (===) which just checks if two values are the same.
== does a whole bunch of other stuff.
It has special handling for functions, special handling for nulls, undefined, strings, you name it.
It get's pretty wacky.
In fact, if you tried to write a function that does what == does it would look something like this:
function isEqual(x, y) { // if `==` were a function
if(typeof y === typeof x) return y === x;
// treat null and undefined the same
var xIsNothing = (y === undefined) || (y === null);
var yIsNothing = (x === undefined) || (x === null);
if(xIsNothing || yIsNothing) return (xIsNothing && yIsNothing);
if(typeof y === "function" || typeof x === "function") {
// if either value is a string
// convert the function into a string and compare
if(typeof x === "string") {
return x === y.toString();
} else if(typeof y === "string") {
return x.toString() === y;
}
return false;
}
if(typeof x === "object") x = toPrimitive(x);
if(typeof y === "object") y = toPrimitive(y);
if(typeof y === typeof x) return y === x;
// convert x and y into numbers if they are not already use the "+" trick
if(typeof x !== "number") x = +x;
if(typeof y !== "number") y = +y;
// actually the real `==` is even more complicated than this, especially in ES6
return x === y;
}
function toPrimitive(obj) {
var value = obj.valueOf();
if(obj !== value) return value;
return obj.toString();
}
So what does this mean?
It means == is complicated.
Because it's complicated it's hard to know what's going to happen when you use it.
Which means you could end up with bugs.
So the moral of the story is...
Make your life less complicated.
Use === instead of ==.
The End.
The === operator is called a strict comparison operator, it does differ from the == operator.
Lets take 2 vars a and b.
For "a == b" to evaluate to true a and b need to be the same value.
In the case of "a === b" a and b must be the same value and also the same type for it to evaluate to true.
Take the following example
var a = 1;
var b = "1";
if (a == b) //evaluates to true as a and b are both 1
{
alert("a == b");
}
if (a === b) //evaluates to false as a is not the same type as b
{
alert("a === b");
}
In summary; using the == operator might evaluate to true in situations where you do not want it to so using the === operator would be safer.
In the 90% usage scenario it won't matter which one you use, but it is handy to know the difference when you get some unexpected behaviour one day.
=== checks same sides are equal in type as well as value.
Example:
'1' === 1 // will return "false" because `string` is not a `number`
Common example:
0 == '' // will be "true", but it's very common to want this check to be "false"
Another common example:
null == undefined // returns "true", but in most cases a distinction is necessary
Many times an untyped check would be handy because you do not care if the value is either undefined, null, 0 or ""
Javascript execution flow diagram for strict equality / Comparison '==='
Javascript execution flow diagram for non strict equality / comparison '=='
JavaScript === vs == .
0==false // true
0===false // false, because they are of a different type
1=="1" // true, auto type coercion
1==="1" // false, because they are of a different type
It means equality without type coercion
type coercion means JavaScript do not automatically convert any other data types to string data types
0==false // true,although they are different types
0===false // false,as they are different types
2=='2' //true,different types,one is string and another is integer but
javaScript convert 2 to string by using == operator
2==='2' //false because by using === operator ,javaScript do not convert
integer to string
2===2 //true because both have same value and same types
In a typical script there will be no performance difference. More important may be the fact that thousand "===" is 1 KB heavier than thousand "==" :) JavaScript profilers can tell you if there is a performance difference in your case.
But personally I would do what JSLint suggests. This recommendation is there not because of performance issues, but because type coercion means ('\t\r\n' == 0) is true.
The equal comparison operator == is confusing and should be avoided.
If you HAVE TO live with it, then remember the following 3 things:
It is not transitive: (a == b) and (b == c) does not lead to (a == c)
It's mutually exclusive to its negation: (a == b) and (a != b) always hold opposite Boolean values, with all a and b.
In case of doubt, learn by heart the following truth table:
EQUAL OPERATOR TRUTH TABLE IN JAVASCRIPT
Each row in the table is a set of 3 mutually "equal" values, meaning that any 2 values among them are equal using the equal == sign*
** STRANGE: note that any two values on the first column are not equal in that sense.**
'' == 0 == false // Any two values among these 3 ones are equal with the == operator
'0' == 0 == false // Also a set of 3 equal values, note that only 0 and false are repeated
'\t' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\r' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\n' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\t\r\n' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
null == undefined // These two "default" values are not-equal to any of the listed values above
NaN // NaN is not equal to any thing, even to itself.
There is unlikely to be any performance difference between the two operations in your usage. There is no type-conversion to be done because both parameters are already the same type. Both operations will have a type comparison followed by a value comparison.
Simply
== means comparison between operands with type coercion
and
=== means comparison between operands without type coercion.
Type coercion in JavaScript means automatically converting data types to other data types.
For example:
123 == "123" // Returns true, because JS coerces string "123" to number 123
// and then goes on to compare `123 == 123`.
123 === "123" // Returns false, because JS does not coerce values of different types here.
Yes! It does matter.
=== operator in javascript checks value as well as type where as == operator just checks the value (does type conversion if required).
You can easily test it. Paste following code in an HTML file and open it in browser
<script>
function onPageLoad()
{
var x = "5";
var y = 5;
alert(x === 5);
};
</script>
</head>
<body onload='onPageLoad();'>
You will get 'false' in alert. Now modify the onPageLoad() method to alert(x == 5); you will get true.
As a rule of thumb, I would generally use === instead of == (and !== instead of !=).
Reasons are explained in in the answers above and also Douglas Crockford is pretty clear about it (JavaScript: The Good Parts).
However there is one single exception:
== null is an efficient way to check for 'is null or undefined':
if( value == null ){
// value is either null or undefined
}
For example jQuery 1.9.1 uses this pattern 43 times, and the JSHint syntax checker even provides the eqnull relaxing option for this reason.
From the jQuery style guide:
Strict equality checks (===) should be used in favor of ==. The only
exception is when checking for undefined and null by way of null.
// Check for both undefined and null values, for some important reason.
undefOrNull == null;
EDIT 2021-03:
Nowadays most browsers
support the Nullish coalescing operator (??)
and the Logical nullish assignment (??=), which allows a more concise way to
assign a default value if a variable is null or undefined, for example:
if (a.speed == null) {
// Set default if null or undefined
a.speed = 42;
}
can be written as any of these forms
a.speed ??= 42;
a.speed ?? a.speed = 42;
a.speed = a.speed ?? 42;
It's a strict check test.
It's a good thing especially if you're checking between 0 and false and null.
For example, if you have:
$a = 0;
Then:
$a==0;
$a==NULL;
$a==false;
All returns true and you may not want this. Let's suppose you have a function that can return the 0th index of an array or false on failure. If you check with "==" false, you can get a confusing result.
So with the same thing as above, but a strict test:
$a = 0;
$a===0; // returns true
$a===NULL; // returns false
$a===false; // returns false
=== operator checks the values as well as the types of the variables for equality.
== operator just checks the value of the variables for equality.
JSLint sometimes gives you unrealistic reasons to modify stuff. === has exactly the same performance as == if the types are already the same.
It is faster only when the types are not the same, in which case it does not try to convert types but directly returns a false.
So, IMHO, JSLint maybe used to write new code, but useless over-optimizing should be avoided at all costs.
Meaning, there is no reason to change == to === in a check like if (a == 'test') when you know it for a fact that a can only be a String.
Modifying a lot of code that way wastes developers' and reviewers' time and achieves nothing.
A simple example is
2 == '2' -> true, values are SAME because of type conversion.
2 === '2' -> false, values are NOT SAME because of no type conversion.
The top 2 answers both mentioned == means equality and === means identity. Unfortunately, this statement is incorrect.
If both operands of == are objects, then they are compared to see if they are the same object. If both operands point to the same object, then the equal operator returns true. Otherwise,
the two are not equal.
var a = [1, 2, 3];
var b = [1, 2, 3];
console.log(a == b) // false
console.log(a === b) // false
In the code above, both == and === get false because a and b are not the same objects.
That's to say: if both operands of == are objects, == behaves same as ===, which also means identity. The essential difference of this two operators is about type conversion. == has conversion before it checks equality, but === does not.
The problem is that you might easily get into trouble since JavaScript have a lot of implicit conversions meaning...
var x = 0;
var isTrue = x == null;
var isFalse = x === null;
Which pretty soon becomes a problem. The best sample of why implicit conversion is "evil" can be taken from this code in MFC / C++ which actually will compile due to an implicit conversion from CString to HANDLE which is a pointer typedef type...
CString x;
delete x;
Which obviously during runtime does very undefined things...
Google for implicit conversions in C++ and STL to get some of the arguments against it...
From the core javascript reference
=== Returns true if the operands are strictly equal (see above)
with no type conversion.
Equality comparison:
Operator ==
Returns true, when both operands are equal. The operands are converted to the same type before being compared.
>>> 1 == 1
true
>>> 1 == 2
false
>>> 1 == '1'
true
Equality and type comparison:
Operator ===
Returns true if both operands are equal and of the same type. It's generally
better and safer if you compare this way, because there's no behind-the-scenes type conversions.
>>> 1 === '1'
false
>>> 1 === 1
true
Here is a handy comparison table that shows the conversions that happen and the differences between == and ===.
As the conclusion states:
"Use three equals unless you fully understand the conversions that take
place for two-equals."
http://dorey.github.io/JavaScript-Equality-Table/
null and undefined are nothingness, that is,
var a;
var b = null;
Here a and b do not have values. Whereas, 0, false and '' are all values. One thing common beween all these are that they are all falsy values, which means they all satisfy falsy conditions.
So, the 0, false and '' together form a sub-group. And on other hand, null & undefined form the second sub-group. Check the comparisons in the below image. null and undefined would equal. The other three would equal to each other. But, they all are treated as falsy conditions in JavaScript.
This is same as any object (like {}, arrays, etc.), non-empty string & Boolean true are all truthy conditions. But, they are all not equal.
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I'm using JSLint to go through JavaScript, and it's returning many suggestions to replace == (two equals signs) with === (three equals signs) when doing things like comparing idSele_UNVEHtype.value.length == 0 inside of an if statement.
Is there a performance benefit to replacing == with ===?
Any performance improvement would be welcomed as many comparison operators exist.
If no type conversion takes place, would there be a performance gain over ==?
The strict equality operator (===) behaves identically to the abstract equality operator (==) except no type conversion is done, and the types must be the same to be considered equal.
Reference: JavaScript Tutorial: Comparison Operators
The == operator will compare for equality after doing any necessary type conversions. The === operator will not do the conversion, so if two values are not the same type === will simply return false. Both are equally quick.
To quote Douglas Crockford's excellent JavaScript: The Good Parts,
JavaScript has two sets of equality operators: === and !==, and their evil twins == and !=. The good ones work the way you would expect. If the two operands are of the same type and have the same value, then === produces true and !== produces false. The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. The rules by which they do that are complicated and unmemorable. These are some of the interesting cases:
'' == '0' // false
0 == '' // true
0 == '0' // true
false == 'false' // false
false == '0' // true
false == undefined // false
false == null // false
null == undefined // true
' \t\r\n ' == 0 // true
The lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use === and !==. All of the comparisons just shown produce false with the === operator.
Update
A good point was brought up by #Casebash in the comments and in #Phillipe Laybaert's answer concerning objects. For objects, == and === act consistently with one another (except in a special case).
var a = [1,2,3];
var b = [1,2,3];
var c = { x: 1, y: 2 };
var d = { x: 1, y: 2 };
var e = "text";
var f = "te" + "xt";
a == b // false
a === b // false
c == d // false
c === d // false
e == f // true
e === f // true
The special case is when you compare a primitive with an object that evaluates to the same primitive, due to its toString or valueOf method. For example, consider the comparison of a string primitive with a string object created using the String constructor.
"abc" == new String("abc") // true
"abc" === new String("abc") // false
Here the == operator is checking the values of the two objects and returning true, but the === is seeing that they're not the same type and returning false. Which one is correct? That really depends on what you're trying to compare. My advice is to bypass the question entirely and just don't use the String constructor to create string objects from string literals.
Reference
https://262.ecma-international.org/5.1/#sec-11.9.3
Using the == operator (Equality)
true == 1; //true, because 'true' is converted to 1 and then compared
"2" == 2; //true, because "2" is converted to 2 and then compared
Using the === operator (Identity)
true === 1; //false
"2" === 2; //false
This is because the equality operator == does type coercion, meaning that the interpreter implicitly tries to convert the values before comparing.
On the other hand, the identity operator === does not do type coercion, and thus does not convert the values when comparing.
Here's an interesting visualisation of the equality comparison between == and ===.
Source: https://github.com/dorey/JavaScript-Equality-Table (demo, unified demo)
var1 === var2
When using === for JavaScript equality testing, everything is as is.
Nothing gets converted before being evaluated.
var1 == var2
When using == for JavaScript equality testing, some funky conversions take place.
Summary of equality in Javascript
Conclusion:
Always use ===, unless you fully understand the funky conversions that take place with ==.
In the answers here, I didn't read anything about what equal means. Some will say that === means equal and of the same type, but that's not really true. It actually means that both operands reference the same object, or in case of value types, have the same value.
So, let's take the following code:
var a = [1,2,3];
var b = [1,2,3];
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
The same here:
var a = { x: 1, y: 2 };
var b = { x: 1, y: 2 };
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
Or even:
var a = { };
var b = { };
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
This behavior is not always obvious. There's more to the story than being equal and being of the same type.
The rule is:
For value types (numbers):
a === b returns true if a and b have the same value and are of the same type
For reference types:
a === b returns true if a and b reference the exact same object
For strings:
a === b returns true if a and b are both strings and contain the exact same characters
Strings: the special case...
Strings are not value types, but in Javascript they behave like value types, so they will be "equal" when the characters in the string are the same and when they are of the same length (as explained in the third rule)
Now it becomes interesting:
var a = "12" + "3";
var b = "123";
alert(a === b); // returns true, because strings behave like value types
But how about this?:
var a = new String("123");
var b = "123";
alert(a === b); // returns false !! (but they are equal and of the same type)
I thought strings behave like value types? Well, it depends who you ask... In this case a and b are not the same type. a is of type Object, while b is of type string. Just remember that creating a string object using the String constructor creates something of type Object that behaves as a string most of the time.
Let me add this counsel:
If in doubt, read the specification!
ECMA-262 is the specification for a scripting language of which JavaScript is a dialect. Of course in practice it matters more how the most important browsers behave than an esoteric definition of how something is supposed to be handled. But it is helpful to understand why new String("a") !== "a".
Please let me explain how to read the specification to clarify this question. I see that in this very old topic nobody had an answer for the very strange effect. So, if you can read a specification, this will help you in your profession tremendously. It is an acquired skill. So, let's continue.
Searching the PDF file for === brings me to page 56 of the specification: 11.9.4. The Strict Equals Operator ( === ), and after wading through the specificationalese I find:
11.9.6 The Strict Equality Comparison Algorithm
The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:
1. If Type(x) is different from Type(y), return false.
2. If Type(x) is Undefined, return true.
3. If Type(x) is Null, return true.
4. If Type(x) is not Number, go to step 11.
5. If x is NaN, return false.
6. If y is NaN, return false.
7. If x is the same number value as y, return true.
8. If x is +0 and y is −0, return true.
9. If x is −0 and y is +0, return true.
10. Return false.
11. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
12. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
13. Return true if x and y refer to the same object or if they refer to objects joined to each other (see 13.1.2). Otherwise, return false.
Interesting is step 11. Yes, strings are treated as value types. But this does not explain why new String("a") !== "a". Do we have a browser not conforming to ECMA-262?
Not so fast!
Let's check the types of the operands. Try it out for yourself by wrapping them in typeof(). I find that new String("a") is an object, and step 1 is used: return false if the types are different.
If you wonder why new String("a") does not return a string, how about some exercise reading a specification? Have fun!
Aidiakapi wrote this in a comment below:
From the specification
11.2.2 The new Operator:
If Type(constructor) is not Object, throw a TypeError exception.
With other words, if String wouldn't be of type Object it couldn't be used with the new operator.
new always returns an Object, even for String constructors, too. And alas! The value semantics for strings (see step 11) is lost.
And this finally means: new String("a") !== "a".
I tested this in Firefox with Firebug using code like this:
console.time("testEquality");
var n = 0;
while (true) {
n++;
if (n == 100000)
break;
}
console.timeEnd("testEquality");
and
console.time("testTypeEquality");
var n = 0;
while (true) {
n++;
if (n === 100000)
break;
}
console.timeEnd("testTypeEquality");
My results (tested five times each and averaged):
==: 115.2
===: 114.4
So I'd say that the miniscule difference (this is over 100000 iterations, remember) is negligible. Performance isn't a reason to do ===. Type safety (well, as safe as you're going to get in JavaScript), and code quality is.
In PHP and JavaScript, it is a strict equality operator. Which means, it will compare both type and values.
In JavaScript it means of the same value and type.
For example,
4 == "4" // will return true
but
4 === "4" // will return false
Why == is so unpredictable?
What do you get when you compare an empty string "" with the number zero 0?
true
Yep, that's right according to == an empty string and the number zero are the same time.
And it doesn't end there, here's another one:
'0' == false // true
Things get really weird with arrays.
[1] == true // true
[] == false // true
[[]] == false // true
[0] == false // true
Then weirder with strings
[1,2,3] == '1,2,3' // true - REALLY?!
'\r\n\t' == 0 // true - Come on!
It get's worse:
When is equal not equal?
let A = '' // empty string
let B = 0 // zero
let C = '0' // zero string
A == B // true - ok...
B == C // true - so far so good...
A == C // **FALSE** - Plot twist!
Let me say that again:
(A == B) && (B == C) // true
(A == C) // **FALSE**
And this is just the crazy stuff you get with primitives.
It's a whole new level of crazy when you use == with objects.
At this point your probably wondering...
Why does this happen?
Well it's because unlike "triple equals" (===) which just checks if two values are the same.
== does a whole bunch of other stuff.
It has special handling for functions, special handling for nulls, undefined, strings, you name it.
It get's pretty wacky.
In fact, if you tried to write a function that does what == does it would look something like this:
function isEqual(x, y) { // if `==` were a function
if(typeof y === typeof x) return y === x;
// treat null and undefined the same
var xIsNothing = (y === undefined) || (y === null);
var yIsNothing = (x === undefined) || (x === null);
if(xIsNothing || yIsNothing) return (xIsNothing && yIsNothing);
if(typeof y === "function" || typeof x === "function") {
// if either value is a string
// convert the function into a string and compare
if(typeof x === "string") {
return x === y.toString();
} else if(typeof y === "string") {
return x.toString() === y;
}
return false;
}
if(typeof x === "object") x = toPrimitive(x);
if(typeof y === "object") y = toPrimitive(y);
if(typeof y === typeof x) return y === x;
// convert x and y into numbers if they are not already use the "+" trick
if(typeof x !== "number") x = +x;
if(typeof y !== "number") y = +y;
// actually the real `==` is even more complicated than this, especially in ES6
return x === y;
}
function toPrimitive(obj) {
var value = obj.valueOf();
if(obj !== value) return value;
return obj.toString();
}
So what does this mean?
It means == is complicated.
Because it's complicated it's hard to know what's going to happen when you use it.
Which means you could end up with bugs.
So the moral of the story is...
Make your life less complicated.
Use === instead of ==.
The End.
The === operator is called a strict comparison operator, it does differ from the == operator.
Lets take 2 vars a and b.
For "a == b" to evaluate to true a and b need to be the same value.
In the case of "a === b" a and b must be the same value and also the same type for it to evaluate to true.
Take the following example
var a = 1;
var b = "1";
if (a == b) //evaluates to true as a and b are both 1
{
alert("a == b");
}
if (a === b) //evaluates to false as a is not the same type as b
{
alert("a === b");
}
In summary; using the == operator might evaluate to true in situations where you do not want it to so using the === operator would be safer.
In the 90% usage scenario it won't matter which one you use, but it is handy to know the difference when you get some unexpected behaviour one day.
=== checks same sides are equal in type as well as value.
Example:
'1' === 1 // will return "false" because `string` is not a `number`
Common example:
0 == '' // will be "true", but it's very common to want this check to be "false"
Another common example:
null == undefined // returns "true", but in most cases a distinction is necessary
Many times an untyped check would be handy because you do not care if the value is either undefined, null, 0 or ""
Javascript execution flow diagram for strict equality / Comparison '==='
Javascript execution flow diagram for non strict equality / comparison '=='
JavaScript === vs == .
0==false // true
0===false // false, because they are of a different type
1=="1" // true, auto type coercion
1==="1" // false, because they are of a different type
It means equality without type coercion
type coercion means JavaScript do not automatically convert any other data types to string data types
0==false // true,although they are different types
0===false // false,as they are different types
2=='2' //true,different types,one is string and another is integer but
javaScript convert 2 to string by using == operator
2==='2' //false because by using === operator ,javaScript do not convert
integer to string
2===2 //true because both have same value and same types
In a typical script there will be no performance difference. More important may be the fact that thousand "===" is 1 KB heavier than thousand "==" :) JavaScript profilers can tell you if there is a performance difference in your case.
But personally I would do what JSLint suggests. This recommendation is there not because of performance issues, but because type coercion means ('\t\r\n' == 0) is true.
The equal comparison operator == is confusing and should be avoided.
If you HAVE TO live with it, then remember the following 3 things:
It is not transitive: (a == b) and (b == c) does not lead to (a == c)
It's mutually exclusive to its negation: (a == b) and (a != b) always hold opposite Boolean values, with all a and b.
In case of doubt, learn by heart the following truth table:
EQUAL OPERATOR TRUTH TABLE IN JAVASCRIPT
Each row in the table is a set of 3 mutually "equal" values, meaning that any 2 values among them are equal using the equal == sign*
** STRANGE: note that any two values on the first column are not equal in that sense.**
'' == 0 == false // Any two values among these 3 ones are equal with the == operator
'0' == 0 == false // Also a set of 3 equal values, note that only 0 and false are repeated
'\t' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\r' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\n' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\t\r\n' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
null == undefined // These two "default" values are not-equal to any of the listed values above
NaN // NaN is not equal to any thing, even to itself.
There is unlikely to be any performance difference between the two operations in your usage. There is no type-conversion to be done because both parameters are already the same type. Both operations will have a type comparison followed by a value comparison.
Simply
== means comparison between operands with type coercion
and
=== means comparison between operands without type coercion.
Type coercion in JavaScript means automatically converting data types to other data types.
For example:
123 == "123" // Returns true, because JS coerces string "123" to number 123
// and then goes on to compare `123 == 123`.
123 === "123" // Returns false, because JS does not coerce values of different types here.
Yes! It does matter.
=== operator in javascript checks value as well as type where as == operator just checks the value (does type conversion if required).
You can easily test it. Paste following code in an HTML file and open it in browser
<script>
function onPageLoad()
{
var x = "5";
var y = 5;
alert(x === 5);
};
</script>
</head>
<body onload='onPageLoad();'>
You will get 'false' in alert. Now modify the onPageLoad() method to alert(x == 5); you will get true.
As a rule of thumb, I would generally use === instead of == (and !== instead of !=).
Reasons are explained in in the answers above and also Douglas Crockford is pretty clear about it (JavaScript: The Good Parts).
However there is one single exception:
== null is an efficient way to check for 'is null or undefined':
if( value == null ){
// value is either null or undefined
}
For example jQuery 1.9.1 uses this pattern 43 times, and the JSHint syntax checker even provides the eqnull relaxing option for this reason.
From the jQuery style guide:
Strict equality checks (===) should be used in favor of ==. The only
exception is when checking for undefined and null by way of null.
// Check for both undefined and null values, for some important reason.
undefOrNull == null;
EDIT 2021-03:
Nowadays most browsers
support the Nullish coalescing operator (??)
and the Logical nullish assignment (??=), which allows a more concise way to
assign a default value if a variable is null or undefined, for example:
if (a.speed == null) {
// Set default if null or undefined
a.speed = 42;
}
can be written as any of these forms
a.speed ??= 42;
a.speed ?? a.speed = 42;
a.speed = a.speed ?? 42;
It's a strict check test.
It's a good thing especially if you're checking between 0 and false and null.
For example, if you have:
$a = 0;
Then:
$a==0;
$a==NULL;
$a==false;
All returns true and you may not want this. Let's suppose you have a function that can return the 0th index of an array or false on failure. If you check with "==" false, you can get a confusing result.
So with the same thing as above, but a strict test:
$a = 0;
$a===0; // returns true
$a===NULL; // returns false
$a===false; // returns false
=== operator checks the values as well as the types of the variables for equality.
== operator just checks the value of the variables for equality.
JSLint sometimes gives you unrealistic reasons to modify stuff. === has exactly the same performance as == if the types are already the same.
It is faster only when the types are not the same, in which case it does not try to convert types but directly returns a false.
So, IMHO, JSLint maybe used to write new code, but useless over-optimizing should be avoided at all costs.
Meaning, there is no reason to change == to === in a check like if (a == 'test') when you know it for a fact that a can only be a String.
Modifying a lot of code that way wastes developers' and reviewers' time and achieves nothing.
A simple example is
2 == '2' -> true, values are SAME because of type conversion.
2 === '2' -> false, values are NOT SAME because of no type conversion.
The top 2 answers both mentioned == means equality and === means identity. Unfortunately, this statement is incorrect.
If both operands of == are objects, then they are compared to see if they are the same object. If both operands point to the same object, then the equal operator returns true. Otherwise,
the two are not equal.
var a = [1, 2, 3];
var b = [1, 2, 3];
console.log(a == b) // false
console.log(a === b) // false
In the code above, both == and === get false because a and b are not the same objects.
That's to say: if both operands of == are objects, == behaves same as ===, which also means identity. The essential difference of this two operators is about type conversion. == has conversion before it checks equality, but === does not.
The problem is that you might easily get into trouble since JavaScript have a lot of implicit conversions meaning...
var x = 0;
var isTrue = x == null;
var isFalse = x === null;
Which pretty soon becomes a problem. The best sample of why implicit conversion is "evil" can be taken from this code in MFC / C++ which actually will compile due to an implicit conversion from CString to HANDLE which is a pointer typedef type...
CString x;
delete x;
Which obviously during runtime does very undefined things...
Google for implicit conversions in C++ and STL to get some of the arguments against it...
From the core javascript reference
=== Returns true if the operands are strictly equal (see above)
with no type conversion.
Equality comparison:
Operator ==
Returns true, when both operands are equal. The operands are converted to the same type before being compared.
>>> 1 == 1
true
>>> 1 == 2
false
>>> 1 == '1'
true
Equality and type comparison:
Operator ===
Returns true if both operands are equal and of the same type. It's generally
better and safer if you compare this way, because there's no behind-the-scenes type conversions.
>>> 1 === '1'
false
>>> 1 === 1
true
Here is a handy comparison table that shows the conversions that happen and the differences between == and ===.
As the conclusion states:
"Use three equals unless you fully understand the conversions that take
place for two-equals."
http://dorey.github.io/JavaScript-Equality-Table/
null and undefined are nothingness, that is,
var a;
var b = null;
Here a and b do not have values. Whereas, 0, false and '' are all values. One thing common beween all these are that they are all falsy values, which means they all satisfy falsy conditions.
So, the 0, false and '' together form a sub-group. And on other hand, null & undefined form the second sub-group. Check the comparisons in the below image. null and undefined would equal. The other three would equal to each other. But, they all are treated as falsy conditions in JavaScript.
This is same as any object (like {}, arrays, etc.), non-empty string & Boolean true are all truthy conditions. But, they are all not equal.