JavaScript If/Else Conditions & Comparisons - Differences

Question

Okay, here's my short question:

I know that === and !== operators will compare the types and then the values, and that == and != will cast the types and then just compare the values.

What about if(myVar) and if(!myVar)?

Is there any difference in the behavior from if(myVar == true) and if(myVar == false)?

Yes, they're different. It's been discussed several times on SO. — , Jun 20 '12 at 17:45
Yes. See these two related questions: http://stackoverflow.com/questions/7496727/why-does-0-a-b-behave-different-than-0-true-a-b and http://stackoverflow.com/questions/5791158/javascript-what-is-the-difference-between-if-x-and-if-x-null — Felix Kling, Jun 20 '12 at 17:45

score 3 · Answer 1 · answered Jun 20 '12 at 17:45

Yes, there is a difference. For example:

if('true' == true) {
    alert("This doesn't happen");
}

if('true') {
    alert("But this does happen.");
}

The reason? They're both converted to numbers for comparison. 'true' is converted to NaN and true is converted to 1.

Avoid this silliness and never write == true or == false.

Felix Kling · Accepted Answer · 2012-06-20T17:59:08.357

Yes, there is a difference. As you already mentioned, if you compare a value with ==, type conversion takes places.

If the values are not of the same type, they will both be converted to either strings or numbers. If one of the values is a boolean and the other is not, both values will be converted to numbers.

The comparison algorithm is defined in section 11.9.3 of the specification. The important step is here:

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

So true is converted to a number first and later myVar will be converted to a number as well.

If you only have if(myVar) though, then the value is converted to a boolean:

2. If ToBoolean(GetValue(exprRef)) is true, then

ToNumber ^[spec] and ToBoolean ^[spec] can return very different results.

Note: If myVar is actually a boolean, then there is no difference between if(myVar == true) and if(myVar).

Thanks, that's the best explanation to me. – headacheCoder Jun 20 '12 at 17:55 — headacheCoder, Jun 20 '12 at 17:55

score 1 · Answer 3 · answered Jun 20 '12 at 17:57

1

Yeah, there is a huge difference in both if(myVar) and if(!myVar) and if(myVar == true) and if(myVar == false)

In if(myVar) and if(!myVar) , !myVar will return true for every "false" value (empty string, 0, null, false, undefined, NaN)

while if(myVar == true) and if(myVar == false) check whether myVar value is true or false. Even if myVar value is NULL, NaN or undefined 0, it'll compare like

if(NULL == true)

Summing up :

NOT operator'!' converts a value into its opposite boolean equivalent. This is different than actually comparing two values.
And if you compare values with '==', JavaScript does type conversion which can lead to unexpected behavior (like undefined == null).

answered Jun 20 '12 at 17:57

Nishu Tayal

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What does *whether it contains anything or not* mean exactly? – Felix Kling Jun 20 '12 at 17:57
@FelixKling It obviously means if it is `null, false, NaN, undefined, "", +0, or -0` :P – Esailija Jun 20 '12 at 18:00
@FelixKling : "false" values i.e. empty string, 0, null, false, undefined, NaN – Nishu Tayal Jun 20 '12 at 18:02
Yeah, I guessed so ;) But the choice of words was not very precise. Sorry for nitpicking. – Felix Kling Jun 20 '12 at 18:06

JavaScript If/Else Conditions & Comparisons - Differences

3 Answers3