The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
null is written with a literal:
null is not an identifier for a property of the global object, like
undefined can be. Instead,
null expresses a lack of identification, indicating that a variable points
to no object. In APIs,
null is often retrieved in a place where an object
can be expected but no object is relevant.
// foo does not exist. It is not defined and has never been initialized: foo; //ReferenceError: foo is not defined
// foo is known to exist now but it has no type or value: var foo = null; foo; //null
When checking for
undefined, beware of the differences
between equality (==) and identity (===) operators, as the former performs
typeof null // "object" (not "null" for legacy reasons) typeof undefined // "undefined" null === undefined // false null == undefined // true null === null // true null == null // true !null // true isNaN(1 + null) // false isNaN(1 + undefined) // true
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