[JS] scope (Introduction)

Study Committee 2021-05-03 13:21:37
js scope introduction


What is the scope ?

Almost all programming languages need to do is “ Variable storage data ”, Otherwise, this programming language will not have much advantage today .

The normal flow of variables storing data is : First create a variable , Then put the value in the variable .

But where do these variables survive ?
How to read the values of these variables ?
How to modify the value of a variable ?
Where can I read , Where can't read ?
Every programming language has a set of rules , And the rule is “ Scope ”.

Use 《 You don't know JavaScript Scroll up 》 A word of :

You need a well-designed set of rules to store variables , And then you can easily find these variables . This set of rules is called scope .

js The scope rules of are not like other languages ( such as c、java etc. ) So rigorous , Even a lot of the time will let the novice confused .

The scope of this article is as follows :

  • Global scope (window / global)
  • Function scope (function)
  • Blocky scope ({})
  • dynamic scope (this A rough explanation )

This paper mainly discusses the daily work need to understand a few points , So I'm not going to go into it Lexical scope And so on .

Variables and constants

Before you know the scope , You need to know js How variables and constants are declared and used .

js The variable declaration for is 2 Kind of :var and let.

Use var perhaps let The syntax for declaring variables is the same , It's just that these two keywords are different .

var a = 123
let b = 456
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var Declared variables , Variable names can be the same , The later statement will override the earlier one . Click to see var usage

let The usage will be more rigorous , yes es6 Bring up the . Use let Declared variables , Cannot repeat declaration . Click to see let usage

js For the time being, only 1 Kind of :const .

const pi = 3.14
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Use const declare constant , Once defined, it cannot be changed , there “ Change ” Can't change the memory address .

So if you declare an object constant , Properties in objects can be changed , Because this does not belong to modify the memory address . Click to see const usage

Global scope

stay window The variables declared under belong to the global scope .

such as

var a = 123
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The opposite is Local scope , such as Function scope Blocky scope etc. , I'll talk about it later .

Be careful

One thing to note , stay js in , If not used var and let Declarative , Strictly speaking, it can't be called a variable . It can only be called window / global Next property .

a = 123
 Copy code 

The above way of writing , and var a = 123 The effect looks similar , But don't use var or let The declared variables are actually mounted on window / global A property on , And attributes can be used delete Delete the , Variables ( Use var and let Statement ) It doesn't work delete Delete .


Function scope

Variables declared inside functions , Belongs to the function scope . It's not accessible outside the function .

function test() {
var a = 123
b = 456
test() // Output 123, Then the output 456
console.log(b) // Output 456
console.log(a) // Report red a is not defined
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In the example above , although b Is in test It's the first time in this function that , But because of b No use var / let / const These keyword definitions , therefore b It's actually attached to window / global Upper .

So you can access... Outside the function b , But I can't access it a .

Blocky scope

stay {} Use let Variables defined and const Defined constant , It's a block level scope .

if (true) {
var a = 123
let b = 456
const c = 789
console.log(a) // Output 123
console.log(b) // Report red b is not defined
console.log(c) // Report red c is not defined
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Use var In this case, the variables defined belong to the global scope .

Use let and const .

dynamic scope

Dynamic scope mainly refers to this .

Dynamic scopes don't care how and where functions and scopes are declared , It only cares where to call ( Except arrow function ).

window.a = 3
function test () {
test.bind({ a: 2 })() // 2
test() // 3
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For another 《 You don't know JavaScript Scroll up 》 Example

function foo() {
console.log(a) // 3( No 2!)
function bar(){
var a = 3
var a = 2
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Because when foo() Unable to find a When you refer to a variable of , Will follow the call stack to call foo() Look for a , Instead of looking up on the nested scope chain of this method .

Extend reading

《 You don't know JavaScript》

article :JavaScript Deep lexical scope and dynamic scope

【JS】 precompile

本文为[Study Committee]所创,转载请带上原文链接,感谢

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