Checking if a key exists in a JavaScript object?

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!

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You should instead use the in operator:

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

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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"

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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

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