ES6 learning (the difference between var let const)

where

Var variable declaration can be re-defined and re-assignment

	var price = 100; //var声明的变量可重新赋值和重复定义
    price = 200;
    console.log(price);

    var price = 100; //var声明的变量可重新赋值和重复定义
    var price = 200;
    console.log(price);

Ends are 200 codes above results in the console display

• var scope: function scope (var variable var variables created in a function can only function in the role, was not created in a function all global variables)

	var price = 100;
    var count = 10;
    if (count > 5) {
      var discount = price * 0.6;
      console.log(`The discount is ${discount}`);  
    }

Results console: 60
Note: In this case the value of the discount directly accessible external braces, because at this time it does not belong to any function, so the global variables

let

• let Scope: block scope (block-level scope, i.e., let variable declarations in curly braces only within the braces)

	var price = 100;
    var count = 10;
    if (count > 5) {
      let discount = price * 0.6;
      console.log(`The discount is ${discount}`);  
    }

At this time, the value of the discount is not directly accessible outside

    var price = 100;
    var count = 10;
    let discount = 90;
    if (count > 5) {
      let discount = price * 0.6;
    }

When the console access discount, find a value of 90. Because if statement let the outside is a global variable, we have direct access to the global variables.

• But can not be reassigned repeat the same scope statement

    let discount = 90;
    discount = 100;

At this time, the console access discount, a value of 100

const

• Scope with let

	var price = 100;
    var count = 10;
    if (count > 5) {
      count discount = price * 0.6;
      console.log(`The discount is ${discount}`);  
    }

At this time, the value of the discount is not directly accessible outside

• Unrepeatable declaration space and can not be reassigned in the same role, but if as an object, you can change the properties of which

	const person = {
      name: 'Jelly',
      age:20
    }
    person.age = 21;

Enter the person in the console, get

{name: "Jelly", age: 21}

If the attribute is not supposed to change, ES5 may be used in Object.freeze () method

    const person = {
      name: 'Jelly',
      age:20
    }
    Object.freeze(person)
    person.age = 21;

In the console input person, get

{name: "Jelly", age: 20}