`null` in JavaScript
In JavaScript, null is a value that represents the intentional absence of any object value.
It is technically a primitive type, although in some cases it behaves as an object. Here's
what you need to know about null:
Checking for null
You can check whether a value is null using the === operator:
if (v === null) {
// Handle `null` case here
}You may also see == null. Using double equals with null is a shorthand for checking whether a value is null or undefined (so-called nullish values).
v == null;
// Equivalent:
v === null || v === undefined;Versus undefined
The JavaScript language spec explicitly defines null as a value that represents the intentional absence of any object value. The difference between null and undefined is the source of some confusion. The primary difference is purely semantic: undefined means the variable has not been assigned a value yet, whereas null means the variable has been explicitly defined as null.
For most practical purposes, null and undefined are often interchangeable as the only two nullish
values. Nullish values are different from non-nullish values in that nullish values throw a TypeError
when you try to access one of their properties, whereas non-nullish values do not.
let v = 42;
v.test; // undefined
v = null;
v.test; // Throws `TypeError: Cannot read property 'test' of null`
v = undefined;
v.test; // Throws `TypeError: Cannot read property 'test' of undefined`In most cases you'll run into as a JavaScript developer, null and undefined are practically
interchangeable. For example, if you're working with MongoDB and Mongoose,
Mongoose stores undefined values as null.
However, there are a few minor differences between how null and undefined work with arithmetic operators and the typeof operator.
With Arithmetic Operators
For the purposes of arithmetic operations, null behaves like 0. If you add, subtract, multiply,
divide, or exponentiate null, JavaScript converts null to 0.
2 + null; // 2
null + 2; // 2
2 - null; // 2
null - 2; // -2
2 * null; // 0
null * 2; // 0
2 ** null; // 1
0 ** 2; // 0
null / 2; // 0This is very different from undefined, which causes all arithmetic operations to result in NaN:
2 + undefined; // NaN
2 * undefined; // NaN
2 - undefined; // NaN
2 ** undefined; // NaNWith typeof
One of the annoying quirks of null is that the typeof operator reports that the type of null is 'object':
typeof null; // 'object'There is a historical reason for this behavior, but it still causes confusion. In order to check if a value is an object and you can access its properties without error, you need a two-step check:
function isObject(v) {
return typeof v === 'object' && v !== null;
}