Checking if an associative array key exists in Javascript

Question

How do I check if a particular key exists in a Javascript associative array?

If a key doesn't exist and I try to access it, will it return false? Or throw an error?

Answer 1

It will return undefined.

var aa = {hello: "world"};
alert( aa["hello"] );      // popup box with "world"
alert( aa["goodbye"] );    // popup boc with "undefined"

undefined is a special constant value. So you can say, e.g.

// note the three equal signs so that null won't be equal to undefined
if( aa["goodbye"] === undefined ) {
    // do something
}

This is probably the best way to check for missing keys. However, as is pointed out in a comment below, it's theoretically possible that you'd want to have the actual value be undefined. I've never needed to do this and can't think of a reason offhand why I'd ever want to, but just for the sake of completeness, you can use the in operator

// this works even if you have {"goodbye": undefined}
if( "goodbye" in aa ) {
    // do something
}

Answer 2

Actually, checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined?

var obj = { key: undefined };
obj["key"] != undefined // false, but the key exists!

You should instead use the in operator:

"key" in obj // true, regardless of the actual value

Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:

obj.hasOwnProperty("key") // true

Answer 3

Of course there are uses for "undefining" a variable. If your logic hinges on its being defined, then you would create an irreversible condition if undefining were disallowed. That would be like saying a Boolean can start out as False, but once True must remain True.