How && works in JavaScript if isn't in a control statement?

Question

Recently I was working with some JavaScript code and I found this:

<script type="text/javascript">
    window.localStorage&&window.localStorage.clear();
</script>

I did research around the web but didn't find anything about how actually that '&&' operator works if isn't in a control statement.

Does anyone know how it works?

gen_Eric · Accepted Answer · 2012-05-25 18:59:15Z

4

That's the same as:

if(window.localStorage){
    window.localStorage.clear();
}

The && short-circuits as soon as it sees a false (or "falsy") value.

So, if window.localStorage is false (or "falsy"), it stops. If it's true, it continues and runs window.localStorage.clear(). The return value is ignored.

answered May 25, 2012 at 18:59

gen_Eric

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2 Comments

Felipe Leñero Over a year ago

Heeey! Thanks a lot, this is a really good trick!! There's another similar tricks?

gen_Eric Over a year ago

@FelipeLeñero: There's ?:. var c = x ? a : b is the same as var c; if(x){c=a}else{c=b}. There's also ||. var d = a || b || c. d will be set to the first not "falsy" value of a, b, and c.

Alec Gorge · Accepted Answer · 2012-05-25 18:59:24Z

1

if this code evaluates to a truthy value&&run this code

answered May 25, 2012 at 18:59

Alec Gorge

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Comments

slowpoison · Accepted Answer · 2012-05-25 19:01:08Z

0

Whether part of a control statement or not, an expression is an expression. In this case the expression involves boolean operations. So the expression will evaluate the second part if first one is true.

In this case it will execute the clear() method on window.localStorage only if window.localStorage is not null.

answered May 25, 2012 at 19:01

slowpoison

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