The difference between null and undefined

First look at javascript Seven types of

1.Undefined( Basic types )

2.Null( Basic types )

3.Boolean( Basic types )

4.String( Basic types )

5.Number( Basic types )

6.Symbol( Basic types )

7.Object( Reference type )

It can be seen that ,Undefined and Null yes Javascript Two special raw data types in (Primary Type), They all have one value , They correspond to each other undefined and null , These two different types of values , It has different semantics and scenes , And show more similar behavior ：

undefined 

undefined Literally ： Undefined value . The meaning of this value is , hope Represents the original state of a variable , Not the result of human operation .  This primitive state will be in the following 4 There are kinds of scenes ：

【1】 Declared a variable , But there is no assignment

1 2	var foo; console.log(foo); // undefined

visit foo, Back to undefined, Indicates that this variable has been declared since , I've never used , No valid values have been defined , That is, in a primitive and unavailable state .

【2】 Access to properties that do not exist on the object  

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console.log(Object.foo); // undefined var arr = []; console.log([0]); // undefined

visit Object On the object foo attribute , return undefined , Express Object Does not exist on or does not define the name foo Properties of . The elements in the array also belong to the object properties inside , To access the subscript is to access this property , return undefined , It means that this element does not exist in the array .

【3】 Function defines parameter , But no arguments were passed

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// Function defines parameter a function fn(a) {     console.log(a); // undefined } fn(); // No arguments passed

function fn The formal parameter is defined a, but fn No parameters passed when called , therefore ,fn Runtime parameters a It's a primitive 、 Unassigned variables .

【4】 Use void Evaluate expressions

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void 0 ; // undefined void false; // undefined void []; // undefined void null; // undefined void function fn(){} ; // undefined

ECMAScript Make it clear void The operator Evaluate any expression to return undefined , This is the same as a function that does not return a value after it performs an operation ,JavaScript All functions in have return values , When there is no return In operation , By default, it returns an original state value , This value right here is undefined, Indicates that the return value of the function is undefined .

therefore ,undefined It usually comes from the original state value of an expression , It's not the result of human operation . Of course , You can also assign a value to a variable manually undefined, But it doesn't make sense , Because a variable that is not assigned is undefined .

null( It is also the end of the object prototype chain )

null Literally, it means ： Null value  . The meaning of this value is , hope Express An object is artificially reset to an empty object , Not the most primitive state of a variable .  The expression in memory is , Variables in the stack don't point to memory objects in the heap , namely ：

Drawing :

When an object is assigned null in the future , The original object is in a free state in memory ,GC It will recycle the object and free the memory . therefore , If you need to release an object , Just set the variable to null, This means that the object has been cleared , Currently invalid . Just imagine , If you put null Switch to undefined Will you feel uncomfortable ? Obviously, the semantics are not clear , Its operation cannot correctly express its desired behavior .

And null Another related question needs to be explained ：

1	typeof null == 'object'

null Have their own type Null, It doesn't belong to Object type ,typeof The reason for this is that Object type , Because JavaScript Data types are represented in binary form at the bottom , The first three bits of binary are 0 Will be typeof Judge as object type , and null The binary bits of are exactly 0 , therefore ,null Be misjudged as Object type .

000 - object , Data is the application of objects

1 - integer , The data is 31 Bit signed integer

010 - Double precision type , The data is double precision

100 - character string , Data is a string

110 - Boolean type , Data is Boolean

Actually , We can get... In another way null The real type of ：

1	Object.prototype.toString.call(null) ; // [object Null]

adopt Object Archetype toString() Method can get JavaScript The real data type of the object in , Of course undefined Types can also be obtained in this way ：

1	Object.prototype.toString.call(undefined) ; // [object Undefined]

similarity

although undefined and null Different semantics and scenarios , But all in all , They all represent a Invalid value .  therefore , stay JS When accessing properties for such values in , All of them will get abnormal results ：

1 2	null.toString(); // Cannot read property 'toString' of null undefined.toString(); // Cannot read property 'toString' of undefined

ECMAScript Normative view , since null and  undefined The behavior is very similar , And they all mean An invalid value , So what they represent is similar , That is to say

1	undefined == null; // true

Don't try to explain this conclusion by converting data types , because ：

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Number(null); // 0 Number(undefined); // NaN // Before comparing equality ,null Not converted to any other type null == 0 ; //false

but === Returns the false , Because full operation === When comparing equality , The data type of the item will not be actively converted , And they don't belong to the same type ：

1 2	undefined === null; // false, Different types undefined !== null;  // true, Different types

summary

In a word, the difference between them is ：undefined It means that a variable is natural 、 The most primitive state value , and null It means that a variable is artificially set as an empty object , Not the original state . therefore , In actual use , To ensure the semantics represented by variables , Don't assign an explicit value to a variable undefined, When you need to release an object , The direct assignment is null that will do .

Detailed explanation undefined And null The difference between - A pixel - Blog Garden