The difference between undefined and null in JavaScript (detailed explanation)

Most computer languages ​​have one and only one value that represents "none", such as NULL in C language, null in Java language, None in Python language, and nil in Ruby language. Somewhat strangely, the JavaScript language actually has two values ​​that represent "nothing": undefined and null. why is that?

1. Similarity

In JavaScript, assign 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

Recently, when I was reading the new book "Speaking JavaScript", I accidentally discovered this problem s answer!

It turns out that this is related to the history of JavaScript. When JavaScript was born in 1995, like Java, only null was set as the value representing "none". According to the tradition of C language, null is designed to be automatically converted to 0.

Number(null)
// 0

5 + null
// 5

However, JavaScript designer Brendan Eich feels that this is not enough, for two reasons.

First of all, null is treated as an object just like in Java. However, JavaScript data types are divided into two categories: primitive types (primitive) and composite types (complex). Brendan Eich feels that the value representing "none" is best not an object.

Secondly, the initial version of JavaScript did not include an 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.

Therefore, Brendan Eich designed another undefined.

3. Initial designThe initial version of JavaScript is distinguished like this: null is an object that represents "none" and is converted into a numerical value. is 0; undefined is a primitive value representing "none", and is NaN when converted to a numerical value.

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

4. Current usageHowever, the above distinction quickly proved to be 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 a parameter of a 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 yet. 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

(End)

Recommended learning:

JavaScript Tutorial

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