Detailed explanation of the difference between undefined and null in JavaScript_Basic knowledge

Somewhat strangely, the JavaScript language actually has two values ​​that represent "none": undefined and null. why is that?

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

Copy code The code is as follows:

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.

Copy code The code is as follows:

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 usage of undefined and null are similar, why do you 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 the answer to this question!
Turns out, this has to do with the history of JavaScript. When JavaScript was born in 1995, initially like Java, only null was set as the value representing "nothing".
According to the tradition of C language, null is designed to be automatically converted to 0.

Copy code The code is as follows:

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.

Copy code The code is as follows:

typeof null
// "object"

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 design
The initial version of JavaScript is distinguished as follows: null is an object that represents "none" and is 0 when converted to a numerical value; undefined is an object that represents "none" Original value, NaN when converted to numeric value.

Copy code The code is as follows:

Number(undefined)
// NaN

5 undefined
// NaN

4. Current usage
However, 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.

Copy code The code is as follows:

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 a 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.

Copy code The code is as follows:

var i;
i // undefined

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

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

var x = f();
x // undefined