I have the following JavaScript code:
var a = 1;
var b = 1;
a === 1 // evaluates to true
b === 1 // evaluates to true
a == b == 1 // evaluates to true
a === b === 1 // evaluates to false
Why does a === b === 1 evaluate to false?
a == b == 1
is evaluated as
((a == b) == 1)
Since a == b is true, the expression becomes
true == 1
Since == does type coercing, it converts true to a number, which becomes 1. So the expression becomes
1 == 1
That is why this expression is true. You can confirm the boolean to number conversion like this
console.log(Number(true));
// 1
console.log(Number(false));
// 0
Similarly,
a === b === 1
is evaluated as
((a === b) === 1)
so
true === 1
Since === doesn't do type coercion (as it is the strict equality operator), this expression is false.
Related
I'm learning TypeScript, and in the getting started page, they talk about how unexpected javascript is.
Source: https://www.typescriptlang.org/docs/handbook/typescript-from-scratch.html
if ("" == 0) {
// It is! But why??
}
if (1 < x < 3) {
// True for *any* value of x!
}
But I still don't understand why 1<x<3 is always true? For example if I let x=10, it will not be true by logic, but why they said it always true?
1 < x < 3 actually is doing this:
(1 < x) < 3
Or even more long form:
const tempVarA = 1 < x
const tempVarB = tempVarA < 3
So 1 < x is either true or false. Then the next step is true < 3 or false < 3. Those don't make much sense as comparisons, but let's see what javascript does with that:
console.log(true < 3) // true
console.log(false < 3) // true
Weird, but let's dig deeper:
console.log(true >= 0) // true
console.log(true >= 1) // true
console.log(true >= 2) // false
console.log(false >= 0) // true
console.log(false >= 1) // false
console.log(false >= 2) // false
It seems that true is being treated as 1 and false is being treated as 0. To verify that let's compare with == (instead of ===) so that it coerces the type of the data for us.
console.log(true == 1) // true
console.log(true == 0) // false
console.log(false == 1) // false
console.log(false == 0) // true
So 1 < x < 3 is always true because false becomes 0 or true becomes 1, and both 0 and 1 always less than 3.
Explanation:
in javascript, comparison operators <, <=, >, >=, ==, and != coerce their operands to make them comparable when they are different types. So when comparing a boolean to a number it coverts the boolean to a number, 0 or 1.
This is why you should almost always use === instead of ==, and why this is a type error in typescript:
const a = true < 3
// Operator '<' cannot be applied to types 'boolean' and 'number'.(2365)
Short version
Javascript and typescript lack a chainable comparison operator.
Did you mean to do this?
1 < x && x < 3
in other words: true is 1 and false is 0. Therefore 1 < x < 3 with x = 5 is as if it executes (1 <5) <3 and writes to 1 <5 = 1 (true) and then 1 <3 = 1 (true). If instead x were 0? Ok 1 <0 is false (0) consequently 0 <3 is true (1)
This question already has answers here:
Why `null >= 0 && null <= 0` but not `null == 0`?
(6 answers)
Closed 4 years ago.
Is the below if statement
if (a >= b)
Equal to this?
if (a > b || a === b)
Or is it equal to this?
if (a > b || a == b)
It is equivalent to if(a > b || a == b)
var a = "2";
var b = 2;
console.log(a >= b); // true
console.log(a > b || a == b); // true (== compares value)
console.log(a > b || a === b); // false (=== compares value and type)
You can test it in the console:
var a = 0;
var b = '0';
a == b; // true
a === b; // false
a >= b; // true
Ergo, >= is equivalent to > || ==.
The actual result depend on the use case if the typeof both a & b is same then (a >= b) is same as (a > b || a === b). This is because == is equality with type coercion
var a = "2";
var b = "2";
console.log(a >= b); // true
console.log(a > b || a == b); // true
console.log(a > b || a === b); // true
var a = "4";
var b = 4;
console.log(a >= b); // true
console.log(a > b || a == b); // true
console.log(a > b || a === b); // false
Actually the first one
if(a >= b)
is similiar to
if(a > b || a == b)
but not equals to
if(a > b || a === b)
because in this last one you are even comparing the type of both the operands.
Example:
x = "5"
console.log(x==parseInt(x)) will return true
console.log(x===parseInt(x)) will return false
So, == does not consider the types of operands.
It depends. If before the statement you had defined a or b with a different type,
if(a > b || a === b)
will return false if the first clause is not true. However if you didn't define a or b before both will have the same type and both expressions are equivalent.
You can understand "===" as
(a == b && sameType(a,b))
I am trying to alert "yes" if ether of the conditions in my if statement are true:
var a = 2;
var b = 1;
if (a = 1 or b = 1 ) {
alert('yes');// should alert in this case
} else {
alert('no');
}
https://jsfiddle.net/90z7urvd/1/
What do I use for the if, if this is possible?
a = 1 will set the value 1 to variable a. It is not doing a comparison. For comparison, you use === or ==
=== (Identity operator) is the correct way to compare if both the types are same.
if (a === 1 || b === 1 ) {
=== operator won't do the type conversion before the comparison while == does the type conversion before the comparison.
For your or case, You may use || operator
var bootresul = someExpression || anotherExpression
Corrected code
var a = 2;
var b = 1;
if (a === 1 || b === 1 ) {
alert('yes');
} else {
alert('no');
}
You are assiging value rather then comparing
Try like this
if (a == 1 || b == 1)
To compare strictly use ===
Like this
if (a === 1 || b === 1)
JSFIDDLE
you can do this
var a = 2;
var b = 1;
if ((a == 1) || (b == 1 )) {
alert('yes');// should alert in this case
} else {
alert('no');
}
the == is one of the relational operator for checking equality and || is a logical operator that is a notion of logical OR
use this to compare just values
if (a == 1 || b == 1){
}
OR use this to compare values and type of variable
if (a === 1 || b === 1){
}
note : == will just check of values and === this will check value with type of variable
var a = 2;
var b = 1;
if (a == 1 || b == 1 ) {
alert('yes');// should alert in this case
} else {
alert('no');
}
I think you were doing assignment instead of comparison
Try using this:
if(a === 1 || b === 1){
alert('YES!')
}else{
alert('NO!')
}
OR you can use ternary operator condition instead of if else
(a == 1 || b == 1) ? alert('YES!') : alert('NO!')
In all of the JavaScript operator precedence charts I can find (like this one and this one), the logical AND (&&) has slightly higher precedence to the logical OR (||).
I can't seem to figure out an expression where the result is different than it would be if they had the same precedence. I figure there must be some way for it to matter or they'd be listed as having the same precedence if it didn't.
For example:
0 || 2 && 0 || 3
is 3, but it doesn't matter how I slice that up, it's always going to be 3:
(0 || 2) && 0 || 3
0 || (2 && 0) || 3
(0 || 2 && 0) || 3
0 || 2 && (0 || 3)
0 || (2 && 0 || 3)
If I make that first 0 something else (like 4), the result is always 4 because the first || doesn't even look at the right-hand side. If I swap the 0 and 3 in the last || around, the result remains 3.
The closest I've come is
0 || false && "" || NaN
...which is NaN, whereas
0 || false && ("" || NaN)
...is false, but I think that's explained purely by the left-to-right semantics, not by && being higher precedence.
I must just be missing it, for what expression does it matter that && has a higher precedence than ||?
true || false && false
is true
(true || false) && false
is false
true || (false && false)
is true
If they had the same precedence and were left-associative, then e.g. the expression
1 || 0 && 2
would be
((1 || 0) && 2) // evaluates to 2
instead of the
(1 || (0 && 2)) // evaluates to 1
that we get from the "usual" precedence rules.
For your structure … || … && … || … (which would be (((… || …) && …) || …) instead of normal ((… || (… && …)) || …)), you'd get different results for values like 0 0 1 0.
Why does the logical AND have slightly higher precedence to the logical OR?
So that the canonical form of boolean expressions, the disjunctive normal form, does not need any parenthesis.
Example:
1 || 0 && 0 -> (1 || 0) && 0 -> (1) && 0 -> 0
1 || 0 && 0 -> 1 || (0 && 0) -> 1 || (0) -> 1
I was expecting the following comparison to give an error:
var A = B = 0;
if(A == B == 0)
console.log(true);
else
console.log(false);
but strangely it returns false.
Even more strangely,
console.log((A == B == 1));
returns true.
How does this "ternary" kind of comparison work?
First, we need to understand that a == comparison between a number and a boolean value will result in internal type conversion of Boolean value to a number (true becomes 1 and false becomes 0)
The expression you have shown is evaluated from left to right. So, first
A == B
is evaluated and the result is true and you are comparing true with 0. Since true becomes 1 during comparison, 1 == 0 evaluates to false. But when you say
console.log((A == B == 1));
A == B is true, which when compared with number, becomes 1 and you are comparing that with 1 again. That is why it prints true.
Assignment operators like = are right-associative: when there is a series of these operators that have the same precedence, they are processed right-to-left, so A = B = 0 is processed as A = (B = 0) (B = 0 returns 0, so both A and B end up as 0).
Equality operators like == are left-associative: same-precedence operators are processed left-to-right. A == B == 0 is processed as (A == B) == 0, and since A == B is true (1), it becomes 1 == 0, which is false (0).
Likewise, A == B == 1 is processed as (A == B) == 1, which becomes 1 == 1, which is true (1).
Source and more info: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence
First, A == B returns true, which is then compared to 0, true == 0 which returns false, or true == 1 which returns true.
It first checks your clause for A == B, which is true, than it starts checking true == 0, and 0 is false. So when you check A == B == 1, you check A==B, which is true, and true == 1. So then it returns true. If you really want to check all possibilities you should do something like this:
if((A==B) && (A==0))
console.log(true);
else
console.log(false);
if((A == B)&& (A== 0)&& (B==0))
console.log(true);
else
console.log(false);
You can work from left to right. In this case you first check if A == B, then you check if this equal to 0. So since A == B, this is true. So now it becomes (true == 0), which is false. If A=1, B=2, then (A == B == 0) would return true! This is because A == B is false and (false == 0) is true!