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?

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 };
console.log(obj["key"] !== undefined); // false, but the key exists!



You should instead use the in operator:



var obj = { key: undefined };
console.log("key" in obj); // true, regardless of the actual value



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



var obj = { not_key: undefined };
console.log(!("key" in obj)); // true if "key" doesn't exist in object
console.log(!"key" in obj); // Do not do this! It is equivalent to "false in obj"



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



var obj = { key: undefined };
console.log(obj.hasOwnProperty("key")); // true



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

Stack Overflow Q#1098040, score: 5614