Thenull keyword refers to thenullprimitive value, which represents the intentional absence of any object value.

Try it

function getVowels(str) {  const m = str.match(/[aeiou]/gi);  if (m === null) {    return 0;  }  return m.length;}console.log(getVowels("sky"));// Expected output: 0

Syntax

null

Description

The keywordnull is a literal for thenull value. Unlikeundefined, which is a global variable,null is not an identifier but a syntax keyword.

null has the following behaviors:

• Likeundefined, accessing any property onnull throws aTypeError instead of returningundefined or searching prototype chains.
• Likeundefined,null is treated asfalsy for boolean operations, andnullish fornullish coalescing andoptional chaining.
• Thetypeof null result is"object". This is a bug in JavaScript that cannot be fixed due to backward compatibility.
• Unlikeundefined,JSON.stringify() can representnull faithfully.

JavaScript is unique to have two nullish values:null andundefined. Semantically, their difference is very minor:undefined represents the absence of a value, whilenull represents the absence of anobject. For example, the end of theprototype chain isnull because the prototype chain is composed of objects;document.querySelector() returnsnull if it doesn't match, because had it matched, the result would be an object. If you are designing an API, you should likely acceptnull andundefined as equivalent inputs, because many codebases have stylistic rules about when to usenull orundefined by default.

Examples

Difference betweennull andundefined

When checking fornull orundefined, beware of thedifferences between equality (==) and identity (===) operators, as the former performs type-conversion.

typeof null; // "object" (not "null" for legacy reasons)typeof undefined; // "undefined"null === undefined; // falsenull == undefined; // truenull === null; // truenull == null; // true!null; // trueNumber.isNaN(1 + null); // falseNumber.isNaN(1 + undefined); // true

Specifications

Browser compatibility

See also

• undefined
• NaN
• void