Detailed explanation of the difference between undefined and null in JavaScript

When we see the title, we all know that the two mean what they mean. We also know that in most computer languages, there is and is only one value that represents "none", for example, NULL in C language , null in Java language, None in Python language, nil in Ruby language, a bit strangely, the JavaScript language actually has two values ​​that represent "none": undefined and null. why is that? Today we will introduce to you the differences between the two!

1. Similarity

In JavaScript, assigning a variable to undefined or null, to be honest, there is almost no difference .

var a = undefined;
var a = null;

In the above code, the a variable is assigned to undefined and null respectively. The two writing methods are almost equivalent.

Undefined and null will be automatically converted to false in the if statement, and the equality operator will even directly report that they are equal.

if (!undefined) 
    console.log('undefined is false');
// undefined is false

if (!null) 
    console.log('null is false');
// null is false

undefined == null
// true

The above code illustrates how similar the behaviors of the two are!

Since the meanings and usages of undefined and null are similar, why do we need to set two such values ​​at the same time? Doesn't this increase the complexity of JavaScript for no reason and trouble beginners? The Dart language, a replacement for the JavaScript language developed by Google, clearly stipulates that there is only null and no undefined!

2. Historical reasons

Number(null)// 0
5 + null// 5

data types are divided into two categories: primitive types (primitive) and synthetic types (complex). Brendan Eich feels that the value representing "none" is best not an object.

error handling mechanism. When a data type mismatch occurs, the type is often automatically converted or fails silently. Brendan Eich feels that if null is automatically converted to 0, it will be difficult to find errors.

3. Initial design

##The initial version of JavaScript is distinguished like this: null It is an object that represents "none" and is 0 when converted to a numerical value; undefined is a primitive value that represents "none" and is NaN when converted to a numerical value.

Number(undefined)// NaN
5 + undefined// NaN

4. Current usage

However, The above distinction quickly proved unfeasible in practice. Currently, null and undefined are basically synonymous, with only some subtle differences.

null means "no object", that is, there should be no value there. Typical usage is:

(1) As the parameter

function, it means that the parameter of the function is not an object. (2) As the end point of the object prototype chain.

Object.getPrototypeOf(Object.prototype)// null

undefined means "missing value", that is, there should be a value here, but it has not been defined. Typical usage is:

(1) When the variable is declared but not assigned a value, it is equal to undefined.

(2) When calling the function, the parameter that should be provided is not provided, and the parameter is equal to undefined.

(3) The object has no assigned attribute, and the value of this attribute is undefined.

(4) When the function does not return a value, it returns undefined by default.

var i;
i // undefined

function f(x){console.log(x)}
f() // undefined

var  o = new Object();
o.p // undefined

var x = f();
x // undefined

Summary: This article introduces the difference between undefined and null in JavaScript from multiple methods. I believe everyone knows this. I hope my understanding and understanding will be helpful to your work!

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