Var, let, const

I. Characteristics of VAR

1. Improvement of existing variables

console.log(a); // undefined var a = 10; Var a; console.log(a); // undefined a = 10;Copy the code

2. A variable can be declared multiple times, and subsequent declarations overwrite previous declarations

var a = 10; var a = 20; console.log(a); / / 20Copy the code

3. When a variable is declared in a function using var, the variable is local

var a = 10; function change(){ var a = 20; } change(); console.log(a); / / 10Copy the code

If you do not use var inside a function, the variable is global

var a = 10; function change(){ a = 20 }; change(); console.log(a); / / 20Copy the code

2. Features of let

1. There is no variable promotion, the variable cannot be used until let declares it (temporary dead zone)

console.log(a); // ReferenceError: a is not defined let
a = 10;
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2. The let command is valid within the code block where the let command resides, and is valid within the block-level scope

{ 
    let a = 10;
}
console.log(a); // ReferenceError: a is not defined
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3. Let does not allow repeated declarations in the same scope. Note that repeated declarations in different scopes do not report errors

let a = 10;
let a = 20; // Uncaught SyntaxError: Identifier 'a' has already been declared 
let a = 10;
{ 
    let a = 20;
} // ok
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Three const.

1. Const declares a read-only variable. Once declared, the value cannot be changed

const a = 10;
a = 20; // TypeError: Assignment to constant variable.
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2. Const must be initialized

const a; // SyntaxError: Missing initializer in const declaration 
const a = 10; // ok
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3. Const does not mean that the value of a variable cannot be changed, but rather that the data stored in the memory address to which the variable points cannot be changed

const obj = { 
    age: 17 
}
obj.age = 18; // ok
obj = { 
    age: 18
} // SyntaxError: Identifier 'obj' has already been declared
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4. Let has all the features of const

4. The difference between

1. Variable ascension

   Var () {var (); var (); var (); var (); var (); var ()

2. Block-level scope

   Var has no block-level scope. Let and const have block-level scope

3. Repeat statement

   Var allows repeated declarations of variables let and const in the same scope do not allow repeated declarations

4. Modify declared variables

   Var and let can declare a read-only constant const. Once declared, the value of the constant cannot be changed, but for reference types such as objects and data, the memory address cannot be changed, but the value inside can be changed.