JavaScript Comparison Operations Null and Undefined
Example
The differences between null and undefined
null and undefined share abstract equality == but not strict equality ===,
null == undefined // true
null === undefined // false
They represent slightly different things:
undefinedrepresents the absence of a value, such as before an identifier/Object property has been created or in the period between identifier/Function parameter creation and it's first set, if any.nullrepresents the intentional absence of a value for an identifier or property which has already been created.
They are different types of syntax:
undefinedis a property of the global Object, usually immutable in the global scope. This means anywhere you can define an identifier other than in the global namespace could hideundefinedfrom that scope (although things can still beundefined)nullis a word literal, so it's meaning can never be changed and attempting to do so will throw an Error.
The similarities between null and undefined
null and undefined are both falsy.
if (null) console.log("won't be logged");
if (undefined) console.log("won't be logged");
Neither null or undefined equal false (see this question).
false == undefined // false
false == null // false
false === undefined // false
false === null // false
Using undefined
- If the current scope can't be trusted, use something which evaluates to undefined, for example
void 0;. - If
undefinedis shadowed by another value, it's just as bad as shadowingArrayorNumber. - Avoid setting something as
undefined. If you want to remove a property bar from an Objectfoo,delete foo.bar;instead. - Existence testing identifier
fooagainstundefinedcould throw a Reference Error, usetypeof fooagainst"undefined"instead.
