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null和undefined的区别.md

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The Difference Between null and undefined

In Js, null and undefined are two basic data types that can be used to represent the concept of "nothing." However, there are differences in semantic expression and practical use.

Description

In most computer languages, there is only one value to represent "nothing," for example, C and C++ use NULL, Java and PHP use null, Python uses None, and lua and Ruby use nil. However, in Js, there are two basic data types, null and undefined, to represent "nothing." In many cases, null and undefined are almost equivalent, for example, in an if statement, both will be automatically converted to false.

var _null = null;
var _undefined = undefined;

if(!_null && !_undefined) console.log("true && true"); // true && true

The == operator considers null and undefined to be equal, but the === operator considers them to be unequal.

console.log(null == undefined); // true
console.log(null === undefined); // false

Using null and undefined to represent "nothing" is due to a historical legacy. Originally, when designing Js, only null was set as the value to represent "nothing." According to the tradition of C language, NULL was designed to be automatically converted to 0. However, the designer of JavaScript, Brendan Eich, felt that this was not enough. Initially, when designing Js, it was thought that null was an Object, which is why typeof(null) === object. Although there have been proposals to change the type of null to typeof(null) === null, it was rejected because it would cause problems with a large number of old Js scripts. Brendan Eich believed that the value representing "nothing" should not be an object, and if null was automatically converted to 0, errors would be harder to detect. Therefore, Brendan Eich designed another data type, undefined. Although null and undefined have a high degree of similarity, they need to be distinguished in terms of semantics and practical use. undefined indicates that the value does not exist, while null indicates that a value has been defined as "empty." Therefore, it is reasonable to set a value to null, for example, obj.v = null;. However, setting a value to undefined is unreasonable because it has already been actively declared, and setting it to undefined means it is undefined.

Differences

  • null is an object representing "nothing," Number(null) === 0, while undefined is a primitive value representing "nothing," Number(undefined) === NaN.
  • null indicates that a value has been defined, but this value is empty.
    • As a function parameter, it indicates that the function's parameter is not an object.
    • As the end of the object prototype chain Object.getPrototypeOf(Object.prototype).
  • undefined indicates that the value is not defined.
    • A variable has been declared but not assigned, represented as undefined.
    • When a function is called and the expected argument is not provided, the parameter value is represented as undefined.
    • The property of an object that has not been assigned a value is represented as undefined.
    • A function without a return value defaults to undefined.

Daily Question

https://github.com/WindrunnerMax/EveryDay

References

https://www.cnblogs.com/sunyang-001/p/10792894.html
http://www.ruanyifeng.com/blog/2014/03/undefined-vs-null.html
https://blog.csdn.net/weixin_39713762/article/details/93807832