Checking if a key exists in a JavaScript object?

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?

Answer

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

If you want to check if a key doesn't exist, remember to use parenthesis:

!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj   // ERROR!  Equivalent to "false in obj"

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

obj.hasOwnProperty("key") // true

For performance comparison between the methods that are in, hasOwnProperty and key is undefined, see this benchmark