The null keyword refers to the null primitive 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 keyword null is a literal for the null value. Unlike undefined, which is a global variable, null is not an identifier but a syntax keyword.

null has the following behaviors:

• Like undefined, accessing any property on null throws a TypeError instead of returning undefined or searching prototype chains.
• Like undefined, null is treated as falsy for boolean operations, and nullish for nullish coalescing and optional chaining.
• The typeof null result is "object". This is a bug in JavaScript that cannot be fixed due to backward compatibility.
• Unlike undefined, JSON.stringify() can represent null faithfully.

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

Examples

Difference between null and undefined

When checking for null or undefined, beware of the differences between equality (==) and identity (===) operators, as the former performs type-conversion.

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
Number.isNaN(1 + null); // false
Number.isNaN(1 + undefined); // true

Specifications

Browser compatibility

See also

• undefined
• NaN
• void