Strange JavaScript Expression

Question

Sometimes, when reading Code written by others, I come across lines of code that I do not understand, for example this one:

 var acceptBtn = document.getElementById('saveBtn');
 (acceptBtn) && (acceptBtn.disabled = false);

What exactly is the meaning of the second line and why is it abbreviated like this?

score 3 · Answer 1 · edited Oct 30 '14 at 13:26

3

It's shorthand for "if acceptBtn is not "falsy", then set it's disabled property to false".

ie:

if(acceptBtn){
    acceptBtn.disabled = false;
}

edited Oct 30 '14 at 13:26

tkone

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answered Oct 30 '14 at 13:24

tymeJV

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Not just `null` values, but anything that evaluates to `false` through type coercion. So `0`, `undefined`, `""`, etc – tkone Oct 30 '14 at 13:27

score 3 · Accepted Answer · answered Oct 30 '14 at 13:25

3

It's a strange syntax, but it's shorthand for:

if(acceptBtn)
    acceptBtn.disabled = false;

I would personally never use it, though.. Bad readability.

answered Oct 30 '14 at 13:25

Thor Jacobsen

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score 2 · Answer 3 · answered Oct 30 '14 at 13:26

The second line takes advantage of the fact that boolean operations in Javascript short-circuit, that is to say if the first part evaluates false then the second part never executes.

It also takes advantage of the fact that javascript can use truthy/falsey values in boolean expressions.

Therefore the second line says, that if acceptBtn is falsey (possibly: null or undefined) then go no further, otherwise set the disabled property to false.

It stops javascript running in to the equivalent to a null-reference exception.

Strange JavaScript Expression

3 Answers3