One 、let command

1. Basic usage

es6 Added let command , Is used to declare variables . Usage and var similar , But the declared variable is only in the let Where the command is located Within code block It works .

  Above picture , stay Code block {} In , Use them separately let and var Two variables declared . Then call these two variables outside the code block ,let Declared variable error ,var The declared variable returns correctly . indicate ,let Declared variables are only valid in the code block in which they reside .

for(let i = 0;i<10;i++){};
console.log(i);  // ReferenceError: i is not defined
// In the above code ,let Declarative i Only in for Circulating efficiency , If it is used outside the circulation, it will report an error .

  The following code uses  let  and  var  To declare the counter  i, The final output is different .

var arr = [];
for (let i = 0; i < 10; i++) {
  arr[i] = function () {
    console.log(i);
  };
}
arr[4](); // 4


var arr = [];
for (var i = 0; i < 10; i++) {
  arr[i] = function () {
    console.log(i);
  };
}
arr[4](); // 10

Variable i Yes, it is var command-declared , Valid globally , Each cycle , Global variables i Will change . in other words , Array arr In every member of i All point to the same i（ That is the last round i Value ）, That is to say 10.

Variable i yes let Declarative , Current i Only valid in this cycle , So every cycle of i It's actually a new variable , Initialize variables of this round i when , On the basis of the last round of calculation .

  in addition ,for The part of the loop that sets the loop variable is a parent scope , Inside the circulatory body is a separate sub scope .

The above code runs correctly , Output 5 Time abc. This means that the variables inside the function i And loop variables i Not in the same scope , Each has its own scope （ The same scope cannot be used let Repeatedly declare the same variable ）.

namely let The declared variable has a block level scope , Valid only in the current scope . 

2. Duplicate statements are not allowed

let Not allowed in the same scope , Repeatedly declare the same variable .var You can declare repeatedly within the same scope .

{
   let a = 10;
   let a = 20;
}
// Report errors  Uncaught SyntaxError: Identifier 'a' has already been declared

{
   var b = 10;
   var b = 20;
} // Don't complain 

3. No variable promotion

Variable Promotion It means that variables can be used before declaration , The value is undefined.var Declared variables have variable promotion , Before declaring, you can use , No mistake. .

{
   console.log(str);  // Output underfined
   var str = 'var Statement ';
}

{
   console.log(bar);  //  Report errors ReferenceError
   let bar = 'let Statement ';
}

In the above code , Variable str use var A statement of order , Variable promotion will occur , When the script starts to run , Variable str It already exists , But it's not worth it , So it will output undefined. Variable bar use let A statement of order , No variable Promotion . This means that before declaring it , Variable bar It doesn't exist , If you use it , Will throw a mistake . 

namely let The declared variable does not have a variable promotion , Only after declaration can you use ,var There are variable promote  

Two 、const command

1. Basic usage

const Declare a read-only constant . Once declared , You can't change the value of a constant .

const str = '123';
str = '456';
//TypeError: Assignment to constant variable.


const name;
//SyntaxError: Missing initializer in const declaration

As shown in the above code , Changing the value of a constant will result in an error . const Once a variable is declared , It must be initialized immediately , It cannot be left for later assignment . Only declare no assignment , You're going to report a mistake .

namely const Used to declare a constant , Must assign immediately . You can't change its value

If const What is declared is data of composite type ( Mainly objects and arrays ), The variable points to the corresponding memory address  , Only the address corresponding to this pointer can be guaranteed to remain unchanged , As for its internal elements, they are variable .

const obj = {name:' Xiao Ming '}
obj.age = 20;

obj = {name:' Little '}
// Uncaught TypeError: Assignment to constant variable.

obj;//{name:' Xiao Ming ',age:20}

  namely const Declare compound constants , The address pointed to does not change , Its internal properties can be changed .

2. Block level scope  

const The scope of let The same command ： Valid only in the block level scope where the declaration is located . 

{
   const value= '123';
}
value;  //VM2767:1 Uncaught ReferenceError: value is not defined

namely const The declared variable has a block level scope , Valid only in the current scope  

3. Duplicate statements are not allowed  

 const And let The same command ： Cannot be declared repeatedly in the same scope . 

var message = 'hello';
const message = '1';
//SyntaxError: Identifier 'message' has already been declared

namely const Not allowed in the same scope , Repeatedly declare the same variable

4. No variable promotion

const And let The same command ： Variable cannot be used before declaration , Will report a mistake .  

{
   console.log(name);
   const name=" Xiaoli "; 
//Uncaught ReferenceError: Cannot access 'name' before initialization
}

namely const The declared variable does not have a variable promotion , Only after declaration can you use .  

summary

let、const and var What's the difference? ？

• let and var Are used to declare variables .const Is used to declare constants .
• const Must be declared with a value , Its value cannot be changed later . What is declared is that a compound variable can change the value of its internal child elements .
• let and const The declared variable has a block level scope , Valid only in the current scope .var There is no block level scope .
• let and const Not allowed in the same scope , Repeatedly declare the same variable .var You can declare repeatedly within the same scope .
• let and const The declared variable does not have a variable promotion , Only after declaration can you use , Otherwise, an error will be reported .var There are variable promote , Before declaration, you can use the value undefined.