The difference between null and undefined in js

The difference between null and undefined in JavaScript

In JavaScript, the difference between null and undefined are special keywords that represent non-existent values. Although they are similar, they have key differences:

1. Definition

• ##null represents a null pointer, pointing to something that does not exist or invalid object.
• undefined Represents an undefined variable whose value has not yet been assigned.

2. Data type

##typeof null
• returns "object". This is a legacy issue in JavaScript, as null was incorrectly classified as an object in its early days.
typeof undefined
• returns "undefined".

3. Value comparison

null
• and undefined are equal when compared ( null == undefined is true). When compared strictly, they are not equal (
null === undefined
• is false).

4. Assignment

##null

You can assign values ​​to variables explicitly.
• undefined
Can only be assigned implicitly, that is, when the variable is undefined.
5. Scope

null

is a global value that can be accessed anywhere.
• undefined
Only makes sense if the variable is not explicitly defined.
6. Usage scenarios

null
• is usually used to express a clear no value. For example, an object that has not yet been created.
undefined
• Usually indicates an undefined variable, such as when a function does not return a value.
Example

// 明确为变量赋值为 null
const myObject = null;

// 未定义变量
let myVariable; // myVariable 为 undefined

Understanding the difference between null

and

undefined is important for writing robust JavaScript code. By distinguishing them, you can avoid common mistakes, such as confusing an undefined variable with null.

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