How do I check if a particular key exists in a JavaScript object or array?
If a key doesn\'t exist, and I try to access it, will it return false? Or throw an error?<
It will return undefined
.
var aa = {hello: "world"};
alert( aa["hello"] ); // popup box with "world"
alert( aa["goodbye"] ); // popup box 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
}