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

<code class="javascript">// 明确为变量赋值为 null
const myObject = null;

// 未定义变量
let myVariable; // myVariable 为 undefined</code>

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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