javascript set a variable if undefined

javascript undefined
pedalpete picture pedalpete · Mar 23, 2011 · Viewed 214.1k times · Source

I know that I can test for a javascript variable and then define it if it is undefined, but is there not some way of saying

var setVariable = localStorage.getItem('value') || 0;

seems like a much clearer way, and I'm pretty sure I've seen this in other languages.

Answer

Alnitak picture Alnitak · Mar 23, 2011

Yes, it can do that, but strictly speaking that will assign the default value if the retrieved value is falsey, as opposed to truly undefined. It would therefore not only match undefined but also null, false, 0, NaN, "" (but not "0").

If you want to set to default only if the variable is strictly undefined then the safest way is to write:

var x = (typeof x === 'undefined') ? your_default_value : x;

On newer browsers it's actually safe to write:

var x = (x === undefined) ? your_default_value : x;

but be aware that it is possible to subvert this on older browsers where it was permitted to declare a variable named undefined that has a defined value, causing the test to fail.

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