JavaScript if(x) vs if(x==true)

Question

In JavaScript , in which cases the following statements won't be logically equal ?

if(x){}

and

if(x==true){}

Thanks

Try `x = 'test'`, as an example. – VisioN Apr 14 '14 at 13:52 — VisioN, Apr 14 '14 at 13:52
`x=' '`, `x=[]`, `x='\n'`, `x='false'`... – nderscore Apr 14 '14 at 13:54 — nderscore, Apr 14 '14 at 13:54

thefourtheye · Accepted Answer · 2014-09-30T12:16:15.690

34

They are not at all equal.

if (x)

checks if x is Truthy where as the later checks if the Boolean value of x is true.

For example,

var x = {};
if (x) {
    console.log("Truthy");
}
if (x == true) {
    console.log("Equal to true");
}

Not only an object, any string (except an empty string), any number (except 0 (because 0 is Falsy) and 1) will be considered as Truthy, but they will not be equal to true.

As per ECMA 5.1 Standards, in if (x), Truthiness of x will be decided, as per the following table

+-----------------------------------------------------------------------+
| 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                                                  |
+-----------------------------------------------------------------------+

Note: The last line object, which includes both objects and Arrays.

But in the later case, as per The Abstract Equality Comparison Algorithm,

If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).

value of x will be converted to a number and that number will be checked against true.

Note:

In JavaScript, true is 1 and false is 0.

console.log(1 == true);
# true
console.log(0 == false);
# true

edited Sep 30 '14 at 12:16

answered Apr 14 '14 at 13:51

thefourtheye

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Mean that Can the later one use string,int other of boolean? – Poomrokc The 3years Apr 14 '14 at 13:52
1

`var x = 'a'; if (x) alert("X"); if (x == true) alert("X true");` This will only alert `"X"` and not `"X true"` – peirix Apr 14 '14 at 13:53
1

1 is considered truthy (from ECMA 5.1: The result is false if the argument is +0, −0, or NaN; otherwise the result is true) – Vitor Tyburski Apr 14 '14 at 14:02
@VitorTyburski Correct. That is why I have mentioned except 0 and 1, in my answer :) – thefourtheye Apr 14 '14 at 14:03
@VitorTyburski I clarified that in the answer now. Please check – thefourtheye Apr 14 '14 at 14:06
@thefourtheye all seems to be ok. But hell, js truthy values are even messier than PHP. – Vitor Tyburski Apr 14 '14 at 14:14
Thanks, I'm accepting this answer, although it's bit hard to comprehend (mostly the loose-comparison rules). Guess because it's indeed messy. – TheZver Apr 14 '14 at 14:21
I'd add rules for array to your table for completeness. I guess that any array is truthy (because it's an object) , but only non-empty arrays equal to true – TheZver Apr 14 '14 at 14:39
@TheZver I think `object` covers all of that, doesn't it? – thefourtheye Apr 14 '14 at 14:40
In JS it is , but It's not generally obvious imho. – TheZver Apr 14 '14 at 14:43
1

@TheZver Fine. I included a note at the bottom of the table :) – thefourtheye Apr 14 '14 at 14:45

score 0 · Answer 2 · answered Apr 14 '14 at 14:03

0

Several cases evaluate to false in the first form, such as empty string, 0, undefined, null.

If you want to be a bit more semantic about it, try the bang bang in front of the expression:

if(!!x){...}

this will convert the expression result to a truthy representing the same semantically. This is closer to an analogue to the expression you describe (x == true)

Also be aware that == is value comparions with type coercion, eg "3" == 3, whereas === asserts equal typing too.

So they are not the same, but often logically represent the same test, thanks to the semantics of the language and the !! you can use

answered Apr 14 '14 at 14:03

Darius

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Thanks. is "if(!!x)" always equal to "if(x)" ? – TheZver Apr 14 '14 at 14:05
No no, imagine this case if(!!"hello") and if("hello" == true), the first expression will evaluate to true because "hello" is not empty string, undefined or null etc so when converted to truthy, is true. Whereas the second expression will be false, because "hello" cannot be coerced to equal the value true. http://jsfiddle.net/CEtN5/ – Darius Apr 14 '14 at 14:09
Sorry I had a mistake in the question. I meant comparison with if(x) – TheZver Apr 14 '14 at 14:10
1

Ok I see your edit. Essentially you are testing the same thing with if(!!x) and if(x), its just the value of !!x is guaranteed to be true or false, whereas if(x) only works because the language evaluates certain conditions to be true/false, as mentioned earlier, 0, empty string, null, undefined.. which aren't strictly truthy, but can be logically interpreted as such – Darius Apr 14 '14 at 14:12
so, to clarify , the following statements are all absolutely equal ? if( x ) , if( !!x ) , if( !!x == true ) , if( !!x === true ) – TheZver Apr 14 '14 at 14:27
1

they will all evaluate to the same thing, so in that sense they are equal... under the hood their efficiency varies I'd imagine – Darius Apr 14 '14 at 14:56

JavaScript if(x) vs if(x==true)

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