javascript - What exactly are expressions that produce *side effects*? - Stack Overflow

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I have trouble understanding this paragraph in the page :

This operator allows inserting expressions that produce side effects into places where an expression that evaluates to undefined is desired.

What exactly are expressions that produce side effects ?

I have trouble understanding this paragraph in the page https://developer.mozilla/en/JavaScript/Reference/Operators/Special/void:

This operator allows inserting expressions that produce side effects into places where an expression that evaluates to undefined is desired.

What exactly are expressions that produce side effects ?

• javascript

Share Improve this question Follow asked Jul 1, 2011 at 1:25 Li JuanLi Juan 1,45722 gold badges1010 silver badges1212 bronze badges 0

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5 Answers 5

Sorted by: Reset to default 6

A function does two things typically: Perform something and return a value. Some functions only do one of those things, some do both. For instance the function:

function square(x) {
  return x * x;
}

Is side-effect free since all it does is return a value and its invocation can always replaced by its return value. On the other hand, something like alert() is only called for its side-effects (alerting a user) and never for its return value.

So what void operator does is it makes JavaScript ignore the return value and state that all you're interested is the function's side-effects.

A simple example is a function call. If you need an "undefined" value, but you also want to call a function that (say) does some DOM manipulation, you can "cast" the result to void and have an undefined result.

I definitely don't think that this would be in the "good parts" of the language, though it does get around the bizarre fact that "undefined" is not really a reserved word. The expression void 0 is definitely going to be really undefined.

The expression (i+=1) evaluates to i+1 but has a side effect of incrementing i by 1.

The purpose of void isn't to mask side effects but specifically for when you want side effects, but don't want the result of the expression.

'Side Effects' is the result of the function that void takes as an argument. In this scenario the function F1 returns 'false', but the wrapping it in 'void' essentially 'swallows' that result or 'side effect':

var F1 = function() { return false; }
void(F1());

Please excuse the gratuitous use of quotes... haha.

One example is this:

<a href="javascript:void(**do stuff here**)">link</a>

The void forces it to return nothing. Without the void potentially some return value will cause the link to take the user away from the page.

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