What is the difference between null and undefined?

null is a keyword in js, indicating a null value; null can be regarded as a special value of object. If the object value is empty, it means that the object is not a valid object;

undefined is not a keyword in js, it is a global variable and a property of Global;

Undefined will be returned in the following situations:

If the variable has been declared but not assigned a value, it is equal to undefined;
When the function is called, no actual parameter is provided, and the parameter is equal to undefined;

function fun1( a ){
    console.log( a )
}

fun1();  //undefined  (形参只是声明了，未传入实参=未赋值)

When using an object property, but the property is not assigned a value or does not exist, the property value is undefined;
When the function does not return a value, it returns undefined by default;

var a = function(){};

console.log(a);   //undefined

1. Similarity

In JavaScript, null and undefined represent "none", and they are both primitive type values and are stored on the stack ;

for example:

Assign variable a to null or undefined, there is almost no difference between the two writing methods;

var a = null;
var a = undefined;

unll and undefined will be automatically converted to false in the if statement;

if(!undefined)
    console.log(" undefined is false ")

        //undefined is false


if(!null)
    console.log(" null is false ")
        
        //null is false




console.log(undefined == null); // true  (作比较时，unll与undefined自动转换为false)

//PS  ： 三等与两等不同；

console.log(undefined === null); // false  (unll为0与undefined为false，三等为false)

2. Difference

1. Different types;

2. It is different when converting the value;

type:

console.log( typeOf undefined ); // undefined

console.log( typeOf null ); // object

Conversion value:

console.log( Number(undefined) ); // NaN

console.log( Number(undefined+10) ); // NaN

console.log( Number(null) ); // 0

console.log( Number(null+10) ); // 10

3. Use

null When a larger object is used up and its memory needs to be released, the object can be set to null.

var arr = [ 111,222,333 ];

arr = null; //Release the reference to the array;

What is the difference between null and undefined?

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