What are the differences between null and undefined in JavaScript

Editor to share with you what are the differences between null and undefined in JavaScript, I believe most people do not know much about it, so share this article for your reference, I hope you can learn a lot after reading this article, let's go to know it!

The difference between null and undefined?

First of all, Undefined and Null are both basic data types, and each of these basic data types has only one value, undefined and null.

The meaning of undefined is undefined, and the meaning of null is empty object (it is not a real object, see the note below! ). Undefined,null is returned when a variable is declared but not yet defined. It is mainly used to assign values to variables that may return an object as initialization.

In fact, null is not an object, although typeof null will output object, but this is only a long-standing Bug of JS. In the original version of JS, a 32-bit system was used to store the type information of variables in a low order for performance reasons. 000 represents an object, but null represents all zeros, so it is mistakenly judged as object. Although the internal type determination code has changed, the Bug has been handed down.

Undefined is not a reserved word in js, which means that we can use undefined as a variable name, which is very dangerous and affects our judgment of undefined values. But there are some ways to get a safe undefined value, such as void 0.

When we use typeof to judge two types, Null typing returns "object", which is a legacy of history. We return true when we compare two types of values using a double equal sign and false when we use three equal signs.

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