Detailed explanation of null and undefined in js

This article mainly shares the detailed explanation of null and undefined in js, including the differences between them. I hope it can help everyone.

1.null is a keyword in JavaScript, which represents a special value and is often used to describe "null value". The typeof data type is object, therefore, null can be considered as a special object. But in fact, null is usually considered to be the only member of its own type.

2. undefined is not a keyword, it indicates a vacancy in the value. It is a value of the variable, indicating that the variable has not been initialized. The typeof data type is undefined. In ECMAScript3, undefined was considered a read-write variable. This error was corrected in ECMAScript5 and changed to read-only.

3. Although null and undefined are different, they both represent "the vacancy of value". Use "==" to consider the two equal, and you need to use the strict equality operator "===" to distinguish them. When the expected value is Boolean, their values ​​are all false, that is, false. Both null and undefined are types that do not have methods.

4. Usage distinction: undefined is a vacancy that represents a system-level, unexpected error-like value. null is a vacancy representing a program-level, normal and expected value. When a function needs to be passed as a parameter, null is more appropriate.

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