What does an exclamation mark before a variable mean in JavaScript - javascript

I'm trying to learn JavaScript by going through some code in an application and I keep seeing !variable in if conditions. For example:
if (!variable.onsubmit || (variable.onsubmit() != false)) {
What is it? Some kind of test if the variable is empty?

! is the logical not operator in JavaScript.
Formally
!expression is read as:
Take expression and evaluate it. In your case that's variable.onsubmit
Case the result of that evaluation and convert it to a boolean. In your case since onsubmit is likely a function, it means - if the function is null or undefined - return false, otherwise return true.
If that evaluation is true, return false. Otherwise return true.
In your case
In your case !variable.onsubmit means return true if there isn't a function defined (and thus is falsy), otherwise return false (since there is a function defined).
Simply put - !variable means take the truth value of variable and negate it.
Thus, if (!variable) { will enter the if clause if variable is false (or coerces to false)
In total
if (!variable.onsubmit || (variable.onsubmit() != false)) {
Means - check if variable.onsubmit is defined and truthy (thus true), then it checks if calling onsubmit returns a result that coerces to true. In a short line it checks if there is no onsubmit or it returns true.
Next time, how do I find this myself?
MDN has a list of operators here.
The language specification specifies such operators, though being the official specification it does contain some jargon which might be hard to understand.

It is a negation operator used for truth tests on a variable.
var myVariable = 1;
if ( ! myVariable )
{
// myVariable evaluates as false
}
if ( myVariable )
{
// myVariable evaluates as true
}

The selected answer already answers the question. One thing to add in this is that ! operator can be used in a rather interesting fashion.
obj = {}
if (!!obj) {console.log('works')} // !!obj = true
obj = undefined
if (!!obj) {console.log('does not work')} // !!obj = false
So double !! will change any expression to a boolean value with no exceptions.
This has a very peculiar use case in some cases.
Lets say there is a function that returns either true or false and nothing else. In that case one could just change return returnValue to return !!returnValue for extra caution (although not need in most of the cases)
Conversely, if a function only accepts true or false and nothing else, one could just call the function as functionA(!!parameter) instead of functionA(parameter), which again is not needed in most of the cases, but could ensure extra security

! is a logic reversal operator, if something was true it will change it to false, if something is false, it will change to true.
example, we know that empty string or 0 in boolean is false.
let a = Boolean("")
console.log(a) // shows false, because empty string in boolean is false
console.log(!a) // shows true, because logic is reversed
easier example:
let a = 5
let b = 1
let c = a > b
console.log(c) // shows true, since a,5 is bigger than b,1
console.log(!c) // shows false, since logic is now reversed

Related

Does if you add two not (!) operators in javascript do they negate each other? [duplicate]

I saw some code that seems to use an operator I don't recognize, in the form of two exclamation points, like so: !!. Can someone please tell me what this operator does?
The context in which I saw this was,
this.vertical = vertical !== undefined ? !!vertical : this.vertical;
It converts Object to boolean. If it was falsey (e.g., 0, null, undefined, etc.), it would be false, otherwise, true.
!object // Inverted Boolean
!!object // Noninverted Boolean, so true Boolean representation
So !! is not an operator; it's just the ! operator twice.
It may be simpler to do:
Boolean(object) // Boolean
Real World Example "Test IE version":
const isIE8 = !! navigator.userAgent.match(/MSIE 8.0/);
console.log(isIE8); // Returns true or false
If you ⇒
console.log(navigator.userAgent.match(/MSIE 8.0/));
// Returns either an Array or null
But if you ⇒
console.log(!!navigator.userAgent.match(/MSIE 8.0/));
// Returns either true or false
It's a horribly obscure way to do a type conversion.
! means NOT. So !true is false, and !false is true. !0 is true, and !1 is false.
So you're converting a value to a Boolean, inverting it, and then inverting it again.
// Maximum Obscurity:
val.enabled = !!userId;
// Partial Obscurity:
val.enabled = (userId != 0) ? true : false;
// And finally, much easier to understand:
val.enabled = (userId != 0);
// Or just
val.enabled = Boolean(userId);
Note: the latter two expressions aren't exactly equivalent to the first expression when it comes to some edge case (when userId is [], for example), due to the way the != operator works and what values are considered truthy.
!!expr (two ! operators followed by an expression) returns a Boolean value (true or false) depending on the truthiness of the expression. It makes more sense when used on non-boolean types. Consider these examples, especially the 3rd example and onward:
!!false === false
!!true === true
!!0 === false
!!parseInt("foo") === false // NaN is falsy
!!1 === true
!!-1 === true // -1 is truthy
!!(1/0) === true // Infinity is truthy
!!"" === false // empty string is falsy
!!"foo" === true // non-empty string is truthy
!!"false" === true // ...even if it contains a falsy value
!!window.foo === false // undefined value is falsy
!!undefined === false // undefined primitive is falsy
!!null === false // null is falsy
!!{} === true // an (empty) object is truthy
!![] === true // an (empty) array is truthy; PHP programmers beware!
Brew some tea:
!! is not an operator. It is the double-use of ! -- which is the logical "not" operator.
In theory:
! determines the "truth" of what a value is not:
The truth is that false is not true (that's why !false results
in true)
The truth is that true is not false (that's why !true results
in false)
!! determines the "truth" of what a value is not not:
The truth is that true is not not true (that's why !!true results in true)
The truth is that false is not not false (that's why !!false results in false)
What we wish to determine in the comparison is the "truth" about the value of a reference, not the value of the reference itself. There is a use-case where we might want to know the truth about a value, even if we expect the value to be false (or falsey), or if we expect the value not to be typeof boolean.
In practice:
Consider a concise function which detects feature functionality (and in this case, platform compatibility) by way of dynamic typing (aka "duck typing"). We want to write a function that returns true if a user's browser supports the HTML5 <audio> element, but we don't want the function to throw an error if <audio> is undefined; and we don't want to use try ... catch to handle any possible errors (because they're gross); and also we don't want to use a check inside the function that won't consistently reveal the truth about the feature (for example, document.createElement('audio') will still create an element called <audio> even if HTML5 <audio> is not supported).
Here are the three approaches:
// this won't tell us anything about HTML5 `<audio>` as a feature
var foo = function(tag, atr) { return document.createElement(tag)[atr]; }
// this won't return true if the feature is detected (although it works just fine)
var bar = function(tag, atr) { return !document.createElement(tag)[atr]; }
// this is the concise, feature-detecting solution we want
var baz = function(tag, atr) { return !!document.createElement(tag)[atr]; }
foo('audio', 'preload'); // returns "auto"
bar('audio', 'preload'); // returns false
baz('audio', 'preload'); // returns true
Each function accepts an argument for a <tag> and an attribute to look for, but they each return different values based on what the comparisons determine.
But wait, there's more!
Some of you probably noticed that in this specific example, one could simply check for a property using the slightly more performant means of checking if the object in question has a property. There are two ways to do this:
// the native `hasOwnProperty` method
var qux = function(tag, atr) { return document.createElement(tag).hasOwnProperty(atr); }
// the `in` operator
var quux = function(tag, atr) { return atr in document.createElement(tag); }
qux('audio', 'preload'); // returns true
quux('audio', 'preload'); // returns true
We digress...
However rare these situations may be, there may exist a few scenarios where the most concise, most performant, and thus most preferred means of getting true from a non-boolean, possibly undefined value is indeed by using !!. Hopefully this ridiculously clears it up.
!! converts the value to the right of it to its equivalent Boolean value. (Think poor man's way of "type-casting".) Its intent is usually to convey to the reader that the code does not care what value is in the variable, but what its "truth" value is.
!!foo applies the unary not operator twice and is used to cast to a Boolean type similar to the use of unary plus +foo to cast to a number and concatenating an empty string ''+foo to cast to a string.
Instead of these hacks, you can also use the constructor functions corresponding to the primitive types (without using new) to explicitly cast values, i.e.,
Boolean(foo) === !!foo
Number(foo) === +foo
String(foo) === ''+foo
So many answers doing half the work. Yes, !!X could be read as "the truthiness of X [represented as a Boolean]". But !! isn't, practically speaking, so important for figuring out whether a single variable is (or even if many variables are) truthy or falsy. !!myVar === true is the same as just myVar. Comparing !!X to a "real" Boolean isn't really useful.
The only thing you gain with !! is the ability to check the truthiness of multiple variables against each other in a repeatable, standardized (and JSLint friendly) fashion.
Simply casting :(
That is...
0 === false is false.
!!0 === false is true.
The above's not so useful. if (!0) gives you the same results as if (!!0 === false). I can't think of a good case for casting a variable to Boolean and then comparing to a "true" Boolean.
See "== and !=" from JSLint's directions (note: Crockford is moving his site around a bit; that link is liable to die at some point) for a little on why:
The == and != operators do type coercion before comparing. This is bad because it causes ' \t\r\n' == 0 to be true. This can mask type errors. JSLint cannot reliably determine if == is being used correctly, so it is best to not use == and != at all and to always use the more reliable === and !== operators instead.
If you only care that a value is truthy or falsy, then use the short form. Instead of
(foo != 0)
just say
(foo)
and instead of
(foo == 0)
say
(!foo)
Note that there are some unintuitive cases where a Boolean will be cast to a number (true is cast to 1 and false to 0) when comparing a Boolean to a number. In this case, !! might be mentally useful. Though, again, these are cases where you're comparing a non-Boolean to a hard-typed Boolean, which is, in my opinion, a serious mistake. if (-1) is still the way to go here.
╔═══════════════════════════════════════╦═══════════════════╦═══════════╗
║ Original ║ Equivalent ║ Result ║
╠═══════════════════════════════════════╬═══════════════════╬═══════════╣
║ if (-1 == true) console.log("spam") ║ if (-1 == 1) ║ undefined ║
║ if (-1 == false) console.log("spam") ║ if (-1 == 0) ║ undefined ║
║ Order doesn't matter... ║ ║ ║
║ if (true == -1) console.log("spam") ║ if (1 == -1) ║ undefined ║
╠═══════════════════════════════════════╬═══════════════════╬═══════════╣
║ if (!!-1 == true) console.log("spam") ║ if (true == true) ║ spam ║ better
╠═══════════════════════════════════════╬═══════════════════╬═══════════╣
║ if (-1) console.log("spam") ║ if (truthy) ║ spam ║ still best
╚═══════════════════════════════════════╩═══════════════════╩═══════════╝
And things get even crazier depending on your engine. WScript, for instance, wins the prize.
function test()
{
return (1 === 1);
}
WScript.echo(test());
Because of some historical Windows jive, that'll output -1 in a message box! Try it in a cmd.exe prompt and see! But WScript.echo(-1 == test()) still gives you 0, or WScript's false. Look away. It's hideous.
Comparing truthiness :)
But what if I have two values I need to check for equal truthiness/falsiness?
Pretend we have myVar1 = 0; and myVar2 = undefined;.
myVar1 === myVar2 is 0 === undefined and is obviously false.
!!myVar1 === !!myVar2 is !!0 === !!undefined and is true! Same truthiness! (In this case, both "have a truthiness of falsy".)
So the only place you'd really need to use "Boolean-cast variables" would be if you had a situation where you're checking if both variables have the same truthiness, right? That is, use !! if you need to see if two variables are both truthy or both falsy (or not), that is, of equal (or not) truthiness.
I can't think of a great, non-contrived use case for that offhand. Maybe you have "linked" fields in a form?
if (!!customerInput.spouseName !== !!customerInput.spouseAge ) {
errorObjects.spouse = "Please either enter a valid name AND age "
+ "for your spouse or leave all spouse fields blank.";
}
So now if you have a truthy for both or a falsy for both spouse name and age, you can continue. Otherwise you've only got one field with a value (or a very early arranged marriage) and need to create an extra error on your errorObjects collection.
Though even in this case, the !! really is superfluous. One ! was enough to cast to a Boolean, and you're just checking equality.
EDIT 24 Oct 2017, 6 Feb 19:
Third-party libraries that expect explicit Boolean values
Here's an interesting case... !! might be useful when third-party libraries expect explicit Boolean values.
React
For instance, False in JSX (React) has a special meaning that's not triggered on simple falsiness. If you tried returning something like the following in your JSX, expecting an int in messageCount...
{messageCount && <div>You have messages!</div>}
... you might be surprised to see React render a 0 when you have zero messages. You have to explicitly return false for JSX not to render. The above statement returns 0, which JSX happily renders, as it should. It can't tell you didn't have Count: {messageCount}.
One fix involves the bangbang, which coerces 0 into !!0, which is false:
{!!messageCount && <div>You have messages!</div>}
JSX' documentation suggests you be more explicit, write self-commenting code, and use a comparison to force to a Boolean.
{messageCount > 0 && <div>You have messages!</div>}
I'm more comfortable handling falsiness myself with a ternary --
{messageCount ? <div>You have messages!</div> : false}
TypeScript
The same deal in TypeScript: If you have a function that returns a Boolean (or you're assigning a value to a Boolean variable), you [usually] can't return/assign a boolean-y value; it has to be a strongly typed boolean. This means, iff myObject is strongly typed, return !myObject; works for a function returning a Boolean, but return myObject; doesn't. You have to return !!myObject (or cast to the proper Boolean another way) to match TypeScript's expectations.
The exception for TypeScript? If myObject was an any, you're back in JavaScript's Wild West and can return it without !!, even if your return type is a Boolean.
Keep in mind that these are JSX and TypeScript conventions, not ones inherent to JavaScript.
But if you see strange 0s in your rendered JSX, think loose falsy management.
It's just the logical NOT operator, twice. It's used to convert something to Boolean, e.g.:
true === !!10
false === !!0
It converts the suffix to a Boolean value.
It's a double not operation. The first ! converts the value to Boolean and inverts its logical value. The second ! inverts the logical value back.
It seems that the !! operator results in a double negation.
var foo = "Hello, World!";
!foo // Result: false
!!foo // Result: true
!! is using the NOT operation twice together. ! converts the value to a Boolean and reverses it, so using it twice, showing the Boolean (false or true) of that value. Here is a simple example to see how !! works:
At first, the place you have:
var zero = 0;
Then you do !0. It will be converted to Boolean and be evaluated to true, because 0 is falsy, so you get the reversed value and converted to Boolean, so it gets evaluated to true.
!zero; //true
But we don't want the reversed Boolean version of the value, so we can reverse it again to get our result! That's why we use another !.
Basically, !! makes us sure the value we get is Boolean, not falsy, truthy, string, etc...
So it's like using the Boolean function in JavaScript, but an easier and shorter way to convert a value to Boolean:
var zero = 0;
!!zero; //false
It simulates the behavior of the Boolean() casting function.
The first NOT returns a Boolean value no matter what operand it is given. The second NOT negates that Boolean value and so gives the true Boolean value of a variable. The end result is the same as using the Boolean() function on a value.
! is "Boolean not", which essentially typecasts the value of "enable" to its boolean opposite. The second ! flips this value. So, !!enable means "not not enable," giving you the value of enable as a Boolean.
I think worth mentioning is that a condition combined with logical AND/OR will not return a Boolean value, but the last success or first fail in case of && and the first success or last fail in case of || of the condition chain.
res = (1 && 2); // res is 2
res = (true && alert) // res is function alert()
res = ('foo' || alert) // res is 'foo'
In order to cast the condition to a true Boolean literal we can use the double negation:
res = !!(1 && 2); // res is true
res = !!(true && alert) // res is true
res = !!('foo' || alert) // res is true
The !! construct is a simple way of turning any JavaScript expression into
its Boolean equivalent.
For example: !!"he shot me down" === true and !!0 === false.
It's not a single operator; it's two. It's equivalent to the following and is a quick way to cast a value to Boolean.
val.enabled = !(!enable);
It forces all things to Boolean.
For example:
console.log(undefined); // -> undefined
console.log(!undefined); // -> true
console.log(!!undefined); // -> false
console.log('abc'); // -> abc
console.log(!'abc'); // -> false
console.log(!!'abc'); // -> true
console.log(0 === false); // -> false
console.log(!0 === false); // -> false
console.log(!!0 === false); // -> true
This question has been answered quite thoroughly, but I'd like to add an answer that I hope is as simplified as possible, making the meaning of !! as simple to grasp as can be.
Because JavaScript has what are called "truthy" and "falsy" values, there are expressions that when evaluated in other expressions will result in a true or false condition, even though the value or expression being examined is not actually true or false.
For instance:
if (document.getElementById('myElement')) {
// Code block
}
If that element does in fact exist, the expression will evaluate as true, and the code block will be executed.
However:
if (document.getElementById('myElement') == true) {
// Code block
}
...will not result in a true condition, and the code block will not be executed, even if the element does exist.
Why? Because document.getElementById() is a "truthy" expression that will evaluate as true in this if() statement, but it is not an actual Boolean value of true.
The double "not" in this case is quite simple. It is simply two nots back to back.
The first one simply "inverts" the truthy or falsy value, resulting in an actual Boolean type, and then the second one "inverts" it back again to its original state, but now in an actual Boolean value. That way you have consistency:
if (!!document.getElementById('myElement')) {}
and
if (!!document.getElementById('myElement') == true) {}
will both return true, as expected.
I suspect this is a leftover from C++ where people override the ! operator, but not the bool operator.
So to get a negative (or positive) answer in that case, you would first need to use the ! operator to get a Boolean, but if you wanted to check the positive case you would use !!.
The if and while statements and the ? operator use truth values to determine which branch of code to run. For example, zero and NaN numbers and the empty string are false, but other numbers and strings are true. Objects are true, but the undefined value and null are both false.
The double negation operator !! calculates the truth value of a value. It's actually two operators, where !!x means !(!x), and behaves as follows:
If x is a false value, !x is true, and !!x is false.
If x is a true value, !x is false, and !!x is true.
When used at the top level of a Boolean context (if, while, or ?), the !! operator is behaviorally a no-op. For example, if (x) and if (!!x) mean the same thing.
Practical uses
However it has several practical uses.
One use is to lossily compress an object to its truth value, so that your code isn't holding a reference to a big object and keeping it alive. Assigning !!some_big_object to a variable instead of some_big_object lets go of it for the garbage collector. This is useful for cases that produce either an object or a false value such as null or the undefined value, such as browser feature detection.
Another use, which I mentioned in an answer about C's corresponding !! operator, is with "lint" tools that look for common typos and print diagnostics. For example, in both C and JavaScript, a few common typos for Boolean operations produce other behaviors whose output isn't quite as Boolean:
if (a = b) is assignment followed by use of the truth value of b; if (a == b) is an equality comparison.
if (a & b) is a bitwise AND; if (a && b) is a logical AND. 2 & 5 is 0 (a false value); 2 && 5 is true.
The !! operator reassures the lint tool that what you wrote is what you meant: do this operation, then take the truth value of the result.
A third use is to produce logical XOR and logical XNOR. In both C and JavaScript, a && b performs a logical AND (true if both sides are true), and a & b performs a bitwise AND. a || b performs a logical OR (true if at least one are true), and a | b performs a bitwise OR. There's a bitwise XOR (exclusive OR) as a ^ b, but there's no built-in operator for logical XOR (true if exactly one side is true). You might, for example, want to allow the user to enter text in exactly one of two fields. What you can do is convert each to a truth value and compare them: !!x !== !!y.
I just wanted to add that
if(variableThing){
// do something
}
is the same as
if(!!variableThing){
// do something
}
But this can be an issue when something is undefined.
// a === undefined, b is an empty object (eg. b.asdf === undefined)
var a, b = {};
// Both of these give error a.foo is not defined etc.
// you'd see the same behavior for !!a.foo and !!b.foo.bar
a.foo
b.foo.bar
// This works -- these return undefined
a && a.foo
b.foo && b.foo.bar
b && b.foo && b.foo.bar
The trick here is the chain of &&s will return the first falsey value it finds -- and this can be fed to an if statement etc. So if b.foo is undefined, it will return undefined and skip the b.foo.bar statement, and we get no error.
The above return undefined but if you have an empty string, false, null, 0, undefined those values will return and soon as we encounter them in the chain -- [] and {} are both "truthy" and we will continue down the so-called "&& chain" to the next value to the right.
P.S. Another way of doing the above (b && b.foo) is (b || {}).foo. Those are equivalent, because if b is undefined then b || {} will be {}, and you'll be accessing a value in an empty object (no error) instead of trying to access a value within "undefined" (causes an error).
So, (b || {}).foo is the same as b && b.foo and ((b || {}).foo || {}).bar is the same as b && b.foo && b.foo.bar.
It is double Boolean negation. It is often used to check if a value is not undefined.
!!x is shorthand for Boolean(x).
The first bang forces the JavaScript engine to run Boolean(x), but it also has the side effect of inverting the value. So the second bang undoes the side effect.
There are tons of great answers here, but if you've read down this far, this helped me to 'get it'. Open the console in Chrome (etc.), and start typing:
!(!(1))
!(!(0))
!(!('truthy'))
!(!(null))
!(!(''))
!(!(undefined))
!(!(new Object())
!(!({}))
woo = 'hoo'
!(!(woo))
...etc., etc., until the light goes on ;)
Naturally, these are all the same as merely typing !!someThing, but the added parentheses might help make it more understandable.
After seeing all these great answers, I would like to add another reason for using !!. Currently I'm working in Angular 2-4 (TypeScript) and I want to return a Boolean as false when my user is not authenticated. If he isn't authenticated, the token-string would be null or "". I can do this by using the next block of code:
public isAuthenticated(): boolean {
return !!this.getToken();
}
It returns the Boolean value of a variable.
Instead, the Boolean class can be used.
(Please read the code descriptions.)
var X = "test"; // The X value is "test" as a String value
var booleanX = !!X // booleanX is `true` as a Boolean value because non-empty strings evaluates as `true` in Boolean
var whatIsXValueInBoolean = Boolean(X) // whatIsXValueInBoolean is `true` again
console.log(Boolean(X) === !!X) // Writes `true`
Namely, Boolean(X) = !!X in use.
Please check code snippet out below ↓
let a = 0
console.log("a: ", a) // Writes a value in its kind
console.log("!a: ", !a) // Writes '0 is NOT true in Boolean' value as Boolean - so that's true. In Boolean, 0 means false and 1 means true.
console.log("!!a: ", !!a) // Writes 0 value in Boolean. 0 means false.
console.log("Boolean(a): ", Boolean(a)) // Equals `!!a`
console.log("\n") // Newline
a = 1
console.log("a: ", a)
console.log("!a: ", !a)
console.log("!!a: ", !!a) // Writes 1 value in Boolean
console.log("\n") // Newline
a = ""
console.log("a: ", a)
console.log("!a: ", !a) // Writes '"" is NOT true in Boolean' value as Boolean - so that's true. In Boolean, empty strings, null and undefined values mean false and if there is a string it means true.
console.log("!!a: ", !!a) // Writes "" value in Boolean
console.log("\n") // Newline
a = "test"
console.log("a: ", a) // Writes a value in its kind
console.log("!a: ", !a)
console.log("!!a: ", !!a) // Writes "test" value in Boolean
console.log("Boolean(a) === !!a: ", Boolean(a) === !!a) // writes true
It is important to remember the evaluations to true and false in JavaScript:
Everything with a "Value" is true (namely truthy), for example:
101,
3.1415,
-11,
"Lucky Brain",
new Object()
and, of course, true
Everything without a "Value" is false (namely falsy), for example:
0,
-0,
"" (empty string),
undefined,
null,
NaN (not a number)
and, of course, false
Applying the "logical not" operator (!) evaluates the operand, converting it to boolean and then negating it. Applying it twice will negate the negation, effectively converting the value to boolean. Not applying the operator will just be a regular assignment of the exact value. Examples:
var value = 23; // number
var valueAsNegatedBoolean = !value; // boolean falsy (because 23 is truthy)
var valueAsBoolean = !!value; // boolean truthy
var copyOfValue = value; // number 23
var value2 = 0;
var value2AsNegatedBoolean = !value2; // boolean truthy (because 0 is falsy)
var value2AsBoolean = !!value2; // boolean falsy
var copyOfValue2 = value2; // number 0
value2 = value; assigns the exact object value even if it is not boolean hence value2 won't necessarily end up being boolean.
value2 = !!value; assigns a guaranteed boolean as the result of the double negation of the operand value and it is equivalent to the following but much shorter and readable:
if (value) {
value2 = true;
} else {
value2 = false;
}
Here is a piece of code from AngularJS:
var requestAnimationFrame = $window.requestAnimationFrame ||
$window.webkitRequestAnimationFrame ||
$window.mozRequestAnimationFrame;
var rafSupported = !!requestAnimationFrame;
Their intention is to set rafSupported to true or false based on the availability of function in requestAnimationFrame.
It can be achieved by checking in the following way in general:
if(typeof requestAnimationFrame === 'function')
rafSupported =true;
else
rafSupported =false;
The short way could be using !!
rafSupported = !!requestAnimationFrame;
So if requestAnimationFrame was assigned a function then !requestAnimationFrame would be false and one more ! of it would be true.
If requestAnimationFrame was assigned undefined then !requestAnimationFrame would be true and one more ! of it would be false.
Use the logical not operator two times.
It means !true = false and !!true = true.

What does return !!this.variable mean? [duplicate]

I saw some code that seems to use an operator I don't recognize, in the form of two exclamation points, like so: !!. Can someone please tell me what this operator does?
The context in which I saw this was,
this.vertical = vertical !== undefined ? !!vertical : this.vertical;
It converts Object to boolean. If it was falsey (e.g., 0, null, undefined, etc.), it would be false, otherwise, true.
!object // Inverted Boolean
!!object // Noninverted Boolean, so true Boolean representation
So !! is not an operator; it's just the ! operator twice.
It may be simpler to do:
Boolean(object) // Boolean
Real World Example "Test IE version":
const isIE8 = !! navigator.userAgent.match(/MSIE 8.0/);
console.log(isIE8); // Returns true or false
If you ⇒
console.log(navigator.userAgent.match(/MSIE 8.0/));
// Returns either an Array or null
But if you ⇒
console.log(!!navigator.userAgent.match(/MSIE 8.0/));
// Returns either true or false
It's a horribly obscure way to do a type conversion.
! means NOT. So !true is false, and !false is true. !0 is true, and !1 is false.
So you're converting a value to a Boolean, inverting it, and then inverting it again.
// Maximum Obscurity:
val.enabled = !!userId;
// Partial Obscurity:
val.enabled = (userId != 0) ? true : false;
// And finally, much easier to understand:
val.enabled = (userId != 0);
// Or just
val.enabled = Boolean(userId);
Note: the latter two expressions aren't exactly equivalent to the first expression when it comes to some edge case (when userId is [], for example), due to the way the != operator works and what values are considered truthy.
!!expr (two ! operators followed by an expression) returns a Boolean value (true or false) depending on the truthiness of the expression. It makes more sense when used on non-boolean types. Consider these examples, especially the 3rd example and onward:
!!false === false
!!true === true
!!0 === false
!!parseInt("foo") === false // NaN is falsy
!!1 === true
!!-1 === true // -1 is truthy
!!(1/0) === true // Infinity is truthy
!!"" === false // empty string is falsy
!!"foo" === true // non-empty string is truthy
!!"false" === true // ...even if it contains a falsy value
!!window.foo === false // undefined value is falsy
!!undefined === false // undefined primitive is falsy
!!null === false // null is falsy
!!{} === true // an (empty) object is truthy
!![] === true // an (empty) array is truthy; PHP programmers beware!
Brew some tea:
!! is not an operator. It is the double-use of ! -- which is the logical "not" operator.
In theory:
! determines the "truth" of what a value is not:
The truth is that false is not true (that's why !false results
in true)
The truth is that true is not false (that's why !true results
in false)
!! determines the "truth" of what a value is not not:
The truth is that true is not not true (that's why !!true results in true)
The truth is that false is not not false (that's why !!false results in false)
What we wish to determine in the comparison is the "truth" about the value of a reference, not the value of the reference itself. There is a use-case where we might want to know the truth about a value, even if we expect the value to be false (or falsey), or if we expect the value not to be typeof boolean.
In practice:
Consider a concise function which detects feature functionality (and in this case, platform compatibility) by way of dynamic typing (aka "duck typing"). We want to write a function that returns true if a user's browser supports the HTML5 <audio> element, but we don't want the function to throw an error if <audio> is undefined; and we don't want to use try ... catch to handle any possible errors (because they're gross); and also we don't want to use a check inside the function that won't consistently reveal the truth about the feature (for example, document.createElement('audio') will still create an element called <audio> even if HTML5 <audio> is not supported).
Here are the three approaches:
// this won't tell us anything about HTML5 `<audio>` as a feature
var foo = function(tag, atr) { return document.createElement(tag)[atr]; }
// this won't return true if the feature is detected (although it works just fine)
var bar = function(tag, atr) { return !document.createElement(tag)[atr]; }
// this is the concise, feature-detecting solution we want
var baz = function(tag, atr) { return !!document.createElement(tag)[atr]; }
foo('audio', 'preload'); // returns "auto"
bar('audio', 'preload'); // returns false
baz('audio', 'preload'); // returns true
Each function accepts an argument for a <tag> and an attribute to look for, but they each return different values based on what the comparisons determine.
But wait, there's more!
Some of you probably noticed that in this specific example, one could simply check for a property using the slightly more performant means of checking if the object in question has a property. There are two ways to do this:
// the native `hasOwnProperty` method
var qux = function(tag, atr) { return document.createElement(tag).hasOwnProperty(atr); }
// the `in` operator
var quux = function(tag, atr) { return atr in document.createElement(tag); }
qux('audio', 'preload'); // returns true
quux('audio', 'preload'); // returns true
We digress...
However rare these situations may be, there may exist a few scenarios where the most concise, most performant, and thus most preferred means of getting true from a non-boolean, possibly undefined value is indeed by using !!. Hopefully this ridiculously clears it up.
!! converts the value to the right of it to its equivalent Boolean value. (Think poor man's way of "type-casting".) Its intent is usually to convey to the reader that the code does not care what value is in the variable, but what its "truth" value is.
!!foo applies the unary not operator twice and is used to cast to a Boolean type similar to the use of unary plus +foo to cast to a number and concatenating an empty string ''+foo to cast to a string.
Instead of these hacks, you can also use the constructor functions corresponding to the primitive types (without using new) to explicitly cast values, i.e.,
Boolean(foo) === !!foo
Number(foo) === +foo
String(foo) === ''+foo
So many answers doing half the work. Yes, !!X could be read as "the truthiness of X [represented as a Boolean]". But !! isn't, practically speaking, so important for figuring out whether a single variable is (or even if many variables are) truthy or falsy. !!myVar === true is the same as just myVar. Comparing !!X to a "real" Boolean isn't really useful.
The only thing you gain with !! is the ability to check the truthiness of multiple variables against each other in a repeatable, standardized (and JSLint friendly) fashion.
Simply casting :(
That is...
0 === false is false.
!!0 === false is true.
The above's not so useful. if (!0) gives you the same results as if (!!0 === false). I can't think of a good case for casting a variable to Boolean and then comparing to a "true" Boolean.
See "== and !=" from JSLint's directions (note: Crockford is moving his site around a bit; that link is liable to die at some point) for a little on why:
The == and != operators do type coercion before comparing. This is bad because it causes ' \t\r\n' == 0 to be true. This can mask type errors. JSLint cannot reliably determine if == is being used correctly, so it is best to not use == and != at all and to always use the more reliable === and !== operators instead.
If you only care that a value is truthy or falsy, then use the short form. Instead of
(foo != 0)
just say
(foo)
and instead of
(foo == 0)
say
(!foo)
Note that there are some unintuitive cases where a Boolean will be cast to a number (true is cast to 1 and false to 0) when comparing a Boolean to a number. In this case, !! might be mentally useful. Though, again, these are cases where you're comparing a non-Boolean to a hard-typed Boolean, which is, in my opinion, a serious mistake. if (-1) is still the way to go here.
╔═══════════════════════════════════════╦═══════════════════╦═══════════╗
║ Original ║ Equivalent ║ Result ║
╠═══════════════════════════════════════╬═══════════════════╬═══════════╣
║ if (-1 == true) console.log("spam") ║ if (-1 == 1) ║ undefined ║
║ if (-1 == false) console.log("spam") ║ if (-1 == 0) ║ undefined ║
║ Order doesn't matter... ║ ║ ║
║ if (true == -1) console.log("spam") ║ if (1 == -1) ║ undefined ║
╠═══════════════════════════════════════╬═══════════════════╬═══════════╣
║ if (!!-1 == true) console.log("spam") ║ if (true == true) ║ spam ║ better
╠═══════════════════════════════════════╬═══════════════════╬═══════════╣
║ if (-1) console.log("spam") ║ if (truthy) ║ spam ║ still best
╚═══════════════════════════════════════╩═══════════════════╩═══════════╝
And things get even crazier depending on your engine. WScript, for instance, wins the prize.
function test()
{
return (1 === 1);
}
WScript.echo(test());
Because of some historical Windows jive, that'll output -1 in a message box! Try it in a cmd.exe prompt and see! But WScript.echo(-1 == test()) still gives you 0, or WScript's false. Look away. It's hideous.
Comparing truthiness :)
But what if I have two values I need to check for equal truthiness/falsiness?
Pretend we have myVar1 = 0; and myVar2 = undefined;.
myVar1 === myVar2 is 0 === undefined and is obviously false.
!!myVar1 === !!myVar2 is !!0 === !!undefined and is true! Same truthiness! (In this case, both "have a truthiness of falsy".)
So the only place you'd really need to use "Boolean-cast variables" would be if you had a situation where you're checking if both variables have the same truthiness, right? That is, use !! if you need to see if two variables are both truthy or both falsy (or not), that is, of equal (or not) truthiness.
I can't think of a great, non-contrived use case for that offhand. Maybe you have "linked" fields in a form?
if (!!customerInput.spouseName !== !!customerInput.spouseAge ) {
errorObjects.spouse = "Please either enter a valid name AND age "
+ "for your spouse or leave all spouse fields blank.";
}
So now if you have a truthy for both or a falsy for both spouse name and age, you can continue. Otherwise you've only got one field with a value (or a very early arranged marriage) and need to create an extra error on your errorObjects collection.
Though even in this case, the !! really is superfluous. One ! was enough to cast to a Boolean, and you're just checking equality.
EDIT 24 Oct 2017, 6 Feb 19:
Third-party libraries that expect explicit Boolean values
Here's an interesting case... !! might be useful when third-party libraries expect explicit Boolean values.
React
For instance, False in JSX (React) has a special meaning that's not triggered on simple falsiness. If you tried returning something like the following in your JSX, expecting an int in messageCount...
{messageCount && <div>You have messages!</div>}
... you might be surprised to see React render a 0 when you have zero messages. You have to explicitly return false for JSX not to render. The above statement returns 0, which JSX happily renders, as it should. It can't tell you didn't have Count: {messageCount}.
One fix involves the bangbang, which coerces 0 into !!0, which is false:
{!!messageCount && <div>You have messages!</div>}
JSX' documentation suggests you be more explicit, write self-commenting code, and use a comparison to force to a Boolean.
{messageCount > 0 && <div>You have messages!</div>}
I'm more comfortable handling falsiness myself with a ternary --
{messageCount ? <div>You have messages!</div> : false}
TypeScript
The same deal in TypeScript: If you have a function that returns a Boolean (or you're assigning a value to a Boolean variable), you [usually] can't return/assign a boolean-y value; it has to be a strongly typed boolean. This means, iff myObject is strongly typed, return !myObject; works for a function returning a Boolean, but return myObject; doesn't. You have to return !!myObject (or cast to the proper Boolean another way) to match TypeScript's expectations.
The exception for TypeScript? If myObject was an any, you're back in JavaScript's Wild West and can return it without !!, even if your return type is a Boolean.
Keep in mind that these are JSX and TypeScript conventions, not ones inherent to JavaScript.
But if you see strange 0s in your rendered JSX, think loose falsy management.
It's just the logical NOT operator, twice. It's used to convert something to Boolean, e.g.:
true === !!10
false === !!0
It converts the suffix to a Boolean value.
It's a double not operation. The first ! converts the value to Boolean and inverts its logical value. The second ! inverts the logical value back.
It seems that the !! operator results in a double negation.
var foo = "Hello, World!";
!foo // Result: false
!!foo // Result: true
!! is using the NOT operation twice together. ! converts the value to a Boolean and reverses it, so using it twice, showing the Boolean (false or true) of that value. Here is a simple example to see how !! works:
At first, the place you have:
var zero = 0;
Then you do !0. It will be converted to Boolean and be evaluated to true, because 0 is falsy, so you get the reversed value and converted to Boolean, so it gets evaluated to true.
!zero; //true
But we don't want the reversed Boolean version of the value, so we can reverse it again to get our result! That's why we use another !.
Basically, !! makes us sure the value we get is Boolean, not falsy, truthy, string, etc...
So it's like using the Boolean function in JavaScript, but an easier and shorter way to convert a value to Boolean:
var zero = 0;
!!zero; //false
It simulates the behavior of the Boolean() casting function.
The first NOT returns a Boolean value no matter what operand it is given. The second NOT negates that Boolean value and so gives the true Boolean value of a variable. The end result is the same as using the Boolean() function on a value.
! is "Boolean not", which essentially typecasts the value of "enable" to its boolean opposite. The second ! flips this value. So, !!enable means "not not enable," giving you the value of enable as a Boolean.
I think worth mentioning is that a condition combined with logical AND/OR will not return a Boolean value, but the last success or first fail in case of && and the first success or last fail in case of || of the condition chain.
res = (1 && 2); // res is 2
res = (true && alert) // res is function alert()
res = ('foo' || alert) // res is 'foo'
In order to cast the condition to a true Boolean literal we can use the double negation:
res = !!(1 && 2); // res is true
res = !!(true && alert) // res is true
res = !!('foo' || alert) // res is true
The !! construct is a simple way of turning any JavaScript expression into
its Boolean equivalent.
For example: !!"he shot me down" === true and !!0 === false.
It's not a single operator; it's two. It's equivalent to the following and is a quick way to cast a value to Boolean.
val.enabled = !(!enable);
It forces all things to Boolean.
For example:
console.log(undefined); // -> undefined
console.log(!undefined); // -> true
console.log(!!undefined); // -> false
console.log('abc'); // -> abc
console.log(!'abc'); // -> false
console.log(!!'abc'); // -> true
console.log(0 === false); // -> false
console.log(!0 === false); // -> false
console.log(!!0 === false); // -> true
This question has been answered quite thoroughly, but I'd like to add an answer that I hope is as simplified as possible, making the meaning of !! as simple to grasp as can be.
Because JavaScript has what are called "truthy" and "falsy" values, there are expressions that when evaluated in other expressions will result in a true or false condition, even though the value or expression being examined is not actually true or false.
For instance:
if (document.getElementById('myElement')) {
// Code block
}
If that element does in fact exist, the expression will evaluate as true, and the code block will be executed.
However:
if (document.getElementById('myElement') == true) {
// Code block
}
...will not result in a true condition, and the code block will not be executed, even if the element does exist.
Why? Because document.getElementById() is a "truthy" expression that will evaluate as true in this if() statement, but it is not an actual Boolean value of true.
The double "not" in this case is quite simple. It is simply two nots back to back.
The first one simply "inverts" the truthy or falsy value, resulting in an actual Boolean type, and then the second one "inverts" it back again to its original state, but now in an actual Boolean value. That way you have consistency:
if (!!document.getElementById('myElement')) {}
and
if (!!document.getElementById('myElement') == true) {}
will both return true, as expected.
I suspect this is a leftover from C++ where people override the ! operator, but not the bool operator.
So to get a negative (or positive) answer in that case, you would first need to use the ! operator to get a Boolean, but if you wanted to check the positive case you would use !!.
The if and while statements and the ? operator use truth values to determine which branch of code to run. For example, zero and NaN numbers and the empty string are false, but other numbers and strings are true. Objects are true, but the undefined value and null are both false.
The double negation operator !! calculates the truth value of a value. It's actually two operators, where !!x means !(!x), and behaves as follows:
If x is a false value, !x is true, and !!x is false.
If x is a true value, !x is false, and !!x is true.
When used at the top level of a Boolean context (if, while, or ?), the !! operator is behaviorally a no-op. For example, if (x) and if (!!x) mean the same thing.
Practical uses
However it has several practical uses.
One use is to lossily compress an object to its truth value, so that your code isn't holding a reference to a big object and keeping it alive. Assigning !!some_big_object to a variable instead of some_big_object lets go of it for the garbage collector. This is useful for cases that produce either an object or a false value such as null or the undefined value, such as browser feature detection.
Another use, which I mentioned in an answer about C's corresponding !! operator, is with "lint" tools that look for common typos and print diagnostics. For example, in both C and JavaScript, a few common typos for Boolean operations produce other behaviors whose output isn't quite as Boolean:
if (a = b) is assignment followed by use of the truth value of b; if (a == b) is an equality comparison.
if (a & b) is a bitwise AND; if (a && b) is a logical AND. 2 & 5 is 0 (a false value); 2 && 5 is true.
The !! operator reassures the lint tool that what you wrote is what you meant: do this operation, then take the truth value of the result.
A third use is to produce logical XOR and logical XNOR. In both C and JavaScript, a && b performs a logical AND (true if both sides are true), and a & b performs a bitwise AND. a || b performs a logical OR (true if at least one are true), and a | b performs a bitwise OR. There's a bitwise XOR (exclusive OR) as a ^ b, but there's no built-in operator for logical XOR (true if exactly one side is true). You might, for example, want to allow the user to enter text in exactly one of two fields. What you can do is convert each to a truth value and compare them: !!x !== !!y.
I just wanted to add that
if(variableThing){
// do something
}
is the same as
if(!!variableThing){
// do something
}
But this can be an issue when something is undefined.
// a === undefined, b is an empty object (eg. b.asdf === undefined)
var a, b = {};
// Both of these give error a.foo is not defined etc.
// you'd see the same behavior for !!a.foo and !!b.foo.bar
a.foo
b.foo.bar
// This works -- these return undefined
a && a.foo
b.foo && b.foo.bar
b && b.foo && b.foo.bar
The trick here is the chain of &&s will return the first falsey value it finds -- and this can be fed to an if statement etc. So if b.foo is undefined, it will return undefined and skip the b.foo.bar statement, and we get no error.
The above return undefined but if you have an empty string, false, null, 0, undefined those values will return and soon as we encounter them in the chain -- [] and {} are both "truthy" and we will continue down the so-called "&& chain" to the next value to the right.
P.S. Another way of doing the above (b && b.foo) is (b || {}).foo. Those are equivalent, because if b is undefined then b || {} will be {}, and you'll be accessing a value in an empty object (no error) instead of trying to access a value within "undefined" (causes an error).
So, (b || {}).foo is the same as b && b.foo and ((b || {}).foo || {}).bar is the same as b && b.foo && b.foo.bar.
It is double Boolean negation. It is often used to check if a value is not undefined.
!!x is shorthand for Boolean(x).
The first bang forces the JavaScript engine to run Boolean(x), but it also has the side effect of inverting the value. So the second bang undoes the side effect.
There are tons of great answers here, but if you've read down this far, this helped me to 'get it'. Open the console in Chrome (etc.), and start typing:
!(!(1))
!(!(0))
!(!('truthy'))
!(!(null))
!(!(''))
!(!(undefined))
!(!(new Object())
!(!({}))
woo = 'hoo'
!(!(woo))
...etc., etc., until the light goes on ;)
Naturally, these are all the same as merely typing !!someThing, but the added parentheses might help make it more understandable.
After seeing all these great answers, I would like to add another reason for using !!. Currently I'm working in Angular 2-4 (TypeScript) and I want to return a Boolean as false when my user is not authenticated. If he isn't authenticated, the token-string would be null or "". I can do this by using the next block of code:
public isAuthenticated(): boolean {
return !!this.getToken();
}
It returns the Boolean value of a variable.
Instead, the Boolean class can be used.
(Please read the code descriptions.)
var X = "test"; // The X value is "test" as a String value
var booleanX = !!X // booleanX is `true` as a Boolean value because non-empty strings evaluates as `true` in Boolean
var whatIsXValueInBoolean = Boolean(X) // whatIsXValueInBoolean is `true` again
console.log(Boolean(X) === !!X) // Writes `true`
Namely, Boolean(X) = !!X in use.
Please check code snippet out below ↓
let a = 0
console.log("a: ", a) // Writes a value in its kind
console.log("!a: ", !a) // Writes '0 is NOT true in Boolean' value as Boolean - so that's true. In Boolean, 0 means false and 1 means true.
console.log("!!a: ", !!a) // Writes 0 value in Boolean. 0 means false.
console.log("Boolean(a): ", Boolean(a)) // Equals `!!a`
console.log("\n") // Newline
a = 1
console.log("a: ", a)
console.log("!a: ", !a)
console.log("!!a: ", !!a) // Writes 1 value in Boolean
console.log("\n") // Newline
a = ""
console.log("a: ", a)
console.log("!a: ", !a) // Writes '"" is NOT true in Boolean' value as Boolean - so that's true. In Boolean, empty strings, null and undefined values mean false and if there is a string it means true.
console.log("!!a: ", !!a) // Writes "" value in Boolean
console.log("\n") // Newline
a = "test"
console.log("a: ", a) // Writes a value in its kind
console.log("!a: ", !a)
console.log("!!a: ", !!a) // Writes "test" value in Boolean
console.log("Boolean(a) === !!a: ", Boolean(a) === !!a) // writes true
It is important to remember the evaluations to true and false in JavaScript:
Everything with a "Value" is true (namely truthy), for example:
101,
3.1415,
-11,
"Lucky Brain",
new Object()
and, of course, true
Everything without a "Value" is false (namely falsy), for example:
0,
-0,
"" (empty string),
undefined,
null,
NaN (not a number)
and, of course, false
Applying the "logical not" operator (!) evaluates the operand, converting it to boolean and then negating it. Applying it twice will negate the negation, effectively converting the value to boolean. Not applying the operator will just be a regular assignment of the exact value. Examples:
var value = 23; // number
var valueAsNegatedBoolean = !value; // boolean falsy (because 23 is truthy)
var valueAsBoolean = !!value; // boolean truthy
var copyOfValue = value; // number 23
var value2 = 0;
var value2AsNegatedBoolean = !value2; // boolean truthy (because 0 is falsy)
var value2AsBoolean = !!value2; // boolean falsy
var copyOfValue2 = value2; // number 0
value2 = value; assigns the exact object value even if it is not boolean hence value2 won't necessarily end up being boolean.
value2 = !!value; assigns a guaranteed boolean as the result of the double negation of the operand value and it is equivalent to the following but much shorter and readable:
if (value) {
value2 = true;
} else {
value2 = false;
}
Here is a piece of code from AngularJS:
var requestAnimationFrame = $window.requestAnimationFrame ||
$window.webkitRequestAnimationFrame ||
$window.mozRequestAnimationFrame;
var rafSupported = !!requestAnimationFrame;
Their intention is to set rafSupported to true or false based on the availability of function in requestAnimationFrame.
It can be achieved by checking in the following way in general:
if(typeof requestAnimationFrame === 'function')
rafSupported =true;
else
rafSupported =false;
The short way could be using !!
rafSupported = !!requestAnimationFrame;
So if requestAnimationFrame was assigned a function then !requestAnimationFrame would be false and one more ! of it would be true.
If requestAnimationFrame was assigned undefined then !requestAnimationFrame would be true and one more ! of it would be false.
Use the logical not operator two times.
It means !true = false and !!true = true.

Why use double exclamation marks in an if statement?

I have recently read some code that uses !! to convert a variable to boolean for evaluation in an if statement. This seems somewhat redundant to me as the variable would be evaluated for its boolean value anyway. Are there any performance benefits to doing this or is it for better browser support?
Sample Code:
var x = 0;
var atTop = x===window.scrollY;
if(!!atTop){
alert("At the top of the page.");
}
Edit:
I have also seen this for operands that are not of boolean types, but I had always assumed that using if would evaluate the boolean value of the variable anyway as all values in Javascript are either "truthy" or "falsey".
Sample Code:
var x = 1;//or any other value including null, undefined, some string, etc
if(!!x){//x is "truthy"
//wouldn't using if(x) be the same???
console.log("something...");
}
Short answer: No, there is no reason.
In your code, it's already a boolean type, there is no need to convert, and convert it back again: you will always get the same result. Actually, if you have a boolean value (true or false), when you use !! with any of them, it will be converted back to its original value:
console.log(!!true); // Will always be "true"
console.log(typeof !!true); // It's still a "boolean" type
console.log(!!false); // Will always be "false"
console.log(typeof !!false); // It stills a "boolean" type
Answer for edited question: Yes, it's the same. That's what if(...) actually does - it's trying to convert any type to boolean.
Here is a small test, you can play around with that and add whatever you want inside the initialArr array in order to test whether it behaves the same with if and !!:
const initialArr = [
undefined,
null,
true,
false,
0,
3,
-1,
+Infinity,
-Infinity,
Infinity,
'any',
'',
function() { return 1 },
{},
{ prop: 1 },
[],
[0],
[0, 1]
];
function testIsTheSame(arr) {
let equolityCounter = 0;
arr.forEach(item => {
let ifStatement = false;
let doubleNotStatement = !!item;
if (item) {
ifStatement = true;
}
if (
ifStatement === doubleNotStatement &&
typeof ifStatement === typeof doubleNotStatement
) {
equolityCounter++;
}
});
console.log(`Is the same: ${equolityCounter === arr.length}`);
}
testIsTheSame(initialArr);
I would say it was mostly done for code-readability. I doubt there is any significant performance nor compatibility implications (someone feel free to test this please)
But in the code-readability aspect, it would imply to me that this variable previously was not a boolean, but we want to evaluate it as one, if you add new logic concerning this variable, keep that in mind.
Although in your case its already boolean, so its 100% redundant, but I'd guess just a habit of someone for the above reasoning applied excessively, and/or seeing it elsewhere and copying that pattern without fully understanding it (short story time: in C# you can name a protected term a variable name by adding #, so var #class = "hello";, a junior dev just assumed all variable names needed # in front of them, and coded that way. I wonder if someone assumed that an if without a comparison operator needs to have !! in front of it as a comparison operator)
There isn't any run-time benefit (or an objective one) whatsoever to doing this in an if statement. The evaluation performed by the if statement will yield precisely the same result as the evaluation produced by double exclamation mark. There is no difference.
Double exclamation marks is a useful expression to ensure boolean value based on truthyness. Consider, for instance:
var text = undefined;
console.log(text); // undefined
console.log(!!text); // false
// It would make sense here, because a boolean value is expected.
var isTextDefined = !!text;
// It would also affect serialization
JSON.stringify(text); // undefined (non-string)
JSON.stringify(!!text); // "false"
I suppose that this is a habit that was picked up because of the above use cases, thus always converting to boolean when wishing to address a variable's truthyness. It's easier for us (humans and programmers) to follow a rule when it's always applied. You'll either use the blinkers in your car all the time, even when there's nobody around, or it's likely that you'll occasionally (or maybe often) forget to signal when you should have.
It all depends on the value of atTop variable. Follow the discussion here.
In summary, if the value of atTop is anything different to a boolean, the first negation will convert it to a boolean value (negated). Second negation will return the boolean value to the equivalent boolean of the original atTop value.
For example, if atTop is undefined, first ! will turn it to boolean true, then the second ! turns it to boolean false (which would be the equivalent boolean of undefined)
However, in your code the atTop value is the result of an strict equality ===, which means you always get a boolean as a result in atTop. Thus, you don't need to convert a non-boolean value to boolean, and so, the !! is not needed.

use of OR in javascript object in my case confusion [duplicate]

This question already has answers here:
Javascript || operator
(5 answers)
Closed 8 years ago.
Question 1
What does options.left = options.left || _KSM.drawOption.left means? I know _KSM.drawOption is referring to the object (also a function), but how does the || operator work here? Does it mean if _KMS.drawOption.left is undefinded, assign options.left to options.left?
Question 2
Why the author didn't use this keyword in this case? I assume it's because in this demo he didn't create an instance, because he just do the calling once. Rigtht? (I've seen a lots of this in jquery plugin that's why I'm consufed when the author call the function name instead of this within a function)
Question 1
This is a way to set a default value to a variable. It actually evaluate the left side and if it is falsy, the variable will be equal to the right side (even if it is also falsy).
That is a bad way of assigning default variable especially when working with integer. 0 is considered as falsy, so you will never be able to assign 0 as a left property. Instead, it will always be the default parameter of _KMS.drawOption .
Question 2
We don't have the full code so we can only assume. But my assumption would be that draw is an event, a function bind with addEventListener. That mean the context (this value) is the target of the event. My guess would be a canvas.
Logical Operators in JavaScript work a bit differently than most people expect.
Logical OR (||)
Returns expr1 if it can be converted to true; otherwise, returns
expr2. Thus, when used with Boolean values, || returns true if either
operand is true; if both are false, returns false.
So instead of the expression resulting in a boolean value, it returns the first value that isn't "falsey". This has the benefit of being a great way to fill in default values in case it is undefined, null, 0, or an empty string.
var myObj = { name: 'Josh' };
myObj.age = myObj.age || 33;
Logical AND (&&)
Returns expr1 if it can be converted to false; otherwise, returns
expr2. Thus, when used with Boolean values, && returns true if both
operands are true; otherwise, returns false.
Basically just the opposite. Since any expression that evaluates to false will short circuit the logical check, it can be useful to test for the existence of a field, before trying to use it.
var myObj = null;
if(myObj && myObj.person && myObj.person.name === "Josh") // false, no error
var myObj = {};
if(myObj && myObj.person && myObj.person.name === "Josh") // false, no error
var myObj = {person: {}};
if(myObj && myObj.person && myObj.person.name === "Josh") // false, no error
var myObj = {person: {name: "Josh"}};
if(myObj && myObj.person && myObj.person.name === "Josh") // true
However! Caution should be taken here because in the case of both boolean values true or false and integer values 0 or anything not 0 you might end up overwriting a legitimate value.
Question 2: Not enough context

boolean in an if statement

Today I've gotten a remark about code considering the way I check whether a variable is true or false in a school assignment.
The code which I had written was something like this:
var booleanValue = true;
function someFunction(){
if(booleanValue === true){
return "something";
}
}
They said it was better/neater to write it like this:
var booleanValue = true;
function someFunction(){
if(booleanValue){
return "something";
}
}
The remark which I have gotten about the "=== true" part was that it was not needed and could create confusion.
However my idea is that it is better to check whether the variable is a boolean or not, especially since Javascript is a loosetyped language.
In the second example a string would also return "something";
So my question; Is it neater to loose the "=== true" part in the future, or is it good practise to check the type of the variable as well.
Edit:
In my "real" code the boolean represents whether an image has been deleted or not, so the only values boolValue should ever have is true or false.
0 and 1 for example shouldn't be in that variable.
First off, the facts:
if (booleanValue)
Will satisfy the if statement for any truthy value of booleanValue including true, any non-zero number, any non-empty string value, any object or array reference, etc...
On the other hand:
if (booleanValue === true)
This will only satisfy the if condition if booleanValue is exactly equal to true. No other truthy value will satisfy it.
On the other hand if you do this:
if (someVar == true)
Then, what Javascript will do is type coerce true to match the type of someVar and then compare the two variables. There are lots of situations where this is likely not what one would intend. Because of this, in most cases you want to avoid == because there's a fairly long set of rules on how Javascript will type coerce two things to be the same type and unless you understand all those rules and can anticipate everything that the JS interpreter might do when given two different types (which most JS developers cannot), you probably want to avoid == entirely.
As an example of how confusing it can be:
var x;
x = 0;
console.log(x == true); // false, as expected
console.log(x == false); // true as expected
x = 1;
console.log(x == true); // true, as expected
console.log(x == false); // false as expected
x = 2;
console.log(x == true); // false, ??
console.log(x == false); // false
For the value 2, you would think that 2 is a truthy value so it would compare favorably to true, but that isn't how the type coercion works. It is converting the right hand value to match the type of the left hand value so its converting true to the number 1 so it's comparing 2 == 1 which is certainly not what you likely intended.
So, buyer beware. It's likely best to avoid == in nearly all cases unless you explicitly know the types you will be comparing and know how all the possible types coercion algorithms work.
So, it really depends upon the expected values for booleanValue and how you want the code to work. If you know in advance that it's only ever going to have a true or false value, then comparing it explicitly with
if (booleanValue === true)
is just extra code and unnecessary and
if (booleanValue)
is more compact and arguably cleaner/better.
If, on the other hand, you don't know what booleanValue might be and you want to test if it is truly set to true with no other automatic type conversions allowed, then
if (booleanValue === true)
is not only a good idea, but required.
For example, if you look at the implementation of .on() in jQuery, it has an optional return value. If the callback returns false, then jQuery will automatically stop propagation of the event. In this specific case, since jQuery wants to ONLY stop propagation if false was returned, they check the return value explicity for === false because they don't want undefined or 0 or "" or anything else that will automatically type-convert to false to also satisfy the comparison.
For example, here's the jQuery event handling callback code:
ret = ( specialHandle || handleObj.handler ).apply( matched.elem, args );
if ( ret !== undefined ) {
event.result = ret;
if ( ret === false ) {
event.preventDefault();
event.stopPropagation();
}
}
You can see that jQuery is explicitly looking for ret === false.
But, there are also many other places in the jQuery code where a simpler check is appropriate given the desire of the code. For example:
// The DOM ready check for Internet Explorer
function doScrollCheck() {
if ( jQuery.isReady ) {
return;
}
...
If you write: if(x === true) , It will be true for only x = true
If you write: if(x) , it will be true for any x that is not: '' (empty string), false, null, undefined, 0, NaN.
In general, it is cleaner and simpler to omit the === true.
However, in Javascript, those statements are different.
if (booleanValue) will execute if booleanValue is truthy – anything other than 0, false, '', NaN, null, and undefined.
if (booleanValue === true) will only execute if booleanValue is precisely equal to true.
In the plain "if" the variable will be coerced to a Boolean and it uses toBoolean on the object:-
Argument Type Result
Undefined false
Null false
Boolean The result equals the input argument (no conversion).
Number The result is false if the argument is +0, −0, or NaN;
otherwise the result is true.
String The result is false if the argument is the empty
String (its length is zero); otherwise the result is true.
Object true.
But comparison with === does not have any type coercion, so they must be equal without coercion.
If you are saying that the object may not even be a Boolean then you may have to consider more than just true/false.
if(x===true){
...
} else if(x===false){
....
} else {
....
}
It depends on your usecase. It may make sense to check the type too, but if it's just a flag, it does not.
If the variable can only ever take on boolean values, then it's reasonable to use the shorter syntax.
If it can potentially be assigned other types, and you need to distinguish true from 1 or "foo", then you must use === true.
The identity (===) operator behaves identically to the equality (==) operator except no type conversion is done, and the types must be the same to be considered equal.
Since the checked value is Boolean it's preferred to use it directly for less coding and at all it did same ==true
Since you already initialized clearly as bool, I think === operator is not required.
I think that your reasoning is sound. But in practice I have found that it is far more common to omit the === comparison. I think that there are three reasons for that:
It does not usually add to the meaning of the expression - that's in cases where the value is known to be boolean anyway.
Because there is a great deal of type-uncertainty in JavaScript, forcing a type check tends to bite you when you get an unexpected undefined or null value. Often you just want your test to fail in such cases. (Though I try to balance this view with the "fail fast" motto).
JavaScript programmers like to play fast-and-loose with types - especially in boolean expressions - because we can.
Consider this example:
var someString = getInput();
var normalized = someString && trim(someString);
// trim() removes leading and trailing whitespace
if (normalized) {
submitInput(normalized);
}
I think that this kind of code is not uncommon. It handles cases where getInput() returns undefined, null, or an empty string. Due to the two boolean evaluations submitInput() is only called if the given input is a string that contains non-whitespace characters.
In JavaScript && returns its first argument if it is falsy or its second argument if the first argument is truthy; so normalized will be undefined if someString was undefined and so forth. That means that none of the inputs to the boolean expressions above are actually boolean values.
I know that a lot of programmers who are accustomed to strong type-checking cringe when seeing code like this. But note applying strong typing would likely require explicit checks for null or undefined values, which would clutter up the code. In JavaScript that is not needed.
In Javascript the idea of boolean is fairly ambiguous. Consider this:
var bool = 0
if(bool){..} //evaluates to false
if(//uninitialized var) //evaluates to false
So when you're using an if statement, (or any other control statement), one does not have to use a "boolean" type var. Therefore, in my opinion, the "=== true" part of your statement is unnecessary if you know it is a boolean, but absolutely necessary if your value is an ambiguous "truthy" var. More on booleans in javscript can be found here.
Also can be tested with Boolean object, if you need to test an object
error={Boolean(errors.email)}
This depends. If you are concerned that your variable could end up as something that resolves to TRUE. Then hard checking is a must. Otherwise it is up to you. However, I doubt that the syntax whatever == TRUE would ever confuse anyone who knew what they were doing.
Revisa https://www.w3schools.com/js/js_comparisons.asp
example:
var p=5;
p==5 ? true
p=="5" ? true
p==="5" ? false
=== means same type also same value
== just same value

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