null and undefined

JavaScript has multiple ways of indicating the absence of a value. This page describes the two most common ways: the null and undefined data types.

null

The null keyword represents an intentionally defined absence of value. null is a primitive, although the typeof operator returns that null is an object. This is an error that has carried over from the first version of JavaScript and been left intentionally unaddressed to avoid breaking expected behavior across the web.

typeof null
> object

You might define a variable as null with the expectation that it reflects either a value assigned to it at some point in a script or an explicitly absent value. You can also assign the null value to an existing reference to clear a previous value.

undefined

undefined is a primitive value assigned to variables that have just been declared, or to the resulting value of an operation that doesn't return a meaningful value. For example, this can happen when you declare a function in a browser's developer console:

function myFunction() {}
> undefined

A function explicitly returns undefined when its return statement returns no value.

(function() {
    return;
}());
> undefined

Comparison of null and undefined

Although undefined and null have some functional overlap, they have different purposes. In the strictest sense, null represents a value intentionally defined as "blank," and undefined represents a lack of any assigned value.

null and undefined are loosely equal, but not strictly equal. The loose equality operator coerces operands of different types to boolean values, making null and undefined both false. The strict equality operator considers operands of different data types to be unequal.

null == undefined
> true

null === undefined
> false

Unlike the reserved keyword null, undefined is a property of the global object. This was a design decision made early in JavaScript's development, and it let legacy browsers overwrite undefined completely. In modern browsers, it's still possible to use undefined as an identifier in non-global scopes, overriding its value within the scope of that declaration. Never use undefined as an identifier. It can cause unexpected behaviors and is likely to confuse future maintainers of your codebase.

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