Checking If a Key Exists in a Javascript Object?

Question

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?

Accepted 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 };
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:

Answer 2

Quick Answer

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?

Accessing directly a missing property using (associative) array style or object style will return an undefined constant.

Must Read

The slow and reliable in operator and hasOwnProperty method

As people have already mentioned here, you could have an object with a property associated with an "undefined" constant.

 var bizzareObj = {valid_key:  undefined};

In that case, you will have to use hasOwnProperty or in operator to know if the key is really there. But, but at what price?

so, I tell you...

in operator and hasOwnProperty are "methods" that use the Property Descriptor mechanism in Javascript (similar to Java reflection in the Java language).

The Property Descriptor type is used to explain the manipulation and reification of named property attributes. Values of the Property Descriptor type are records composed of named fields where each field’s name is an attribute name and its value is a corresponding attribute value as specified in 8.6.1. In addition, any field may be present or absent.

On the other hand, calling an object method or key will use Javascript [[Get]] mechanism. That is a far way faster!

Checking If a Key Exists in a Javascript Object?

31 Answers

Quick Answer

Must Read

The slow and reliable in operator and hasOwnProperty method

James H. Sterling