ES6 related to grammar

es6.ruanyifeng.com/#docs/intro

1, let the command and const

• let
  • let define variables, variables can not be redefined, but can change its value
  • Having a block-level scope
  • No variable lift, you must first define and then use
  • Variables let statement is not pressed to the window object, independent
• const
  • Use const keyword to define constants
  • Constants are immutable, once defined, its value can not be modified
  • When initialization constants must be given an initial value
  • Having a block-level scope
  • No variable lift, you must first define and then use
  • Constant also independent, the definition is not pressed into window object

2, deconstruction assignment

• Deconstruction array assignment

let arr = [1, 2, 3];
let [a, b, c] = arr;
console.log(a, b, c); // 输出 1 2 3
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• Deconstruction object assignment

let { foo, bar } = {foo: 'aaa', bar: 'bbb'};
console.log(foo, bar); // aaa, bbb

let {a, c} = {a: 'hello', b: 'world'};
console.log(a, c); // hello, undefined
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• Deconstruction string assignment
• Deconstructing the assignment of function parameters

3, arrow function

// 非箭头函数
let fn = function (x) {
    return x * 2;
}
// 箭头函数，等同于上面的函数
let fn = (x) => {
    return x * 2;
}
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• Features arrow function
  • Only one parameter, parentheses may be omitted
  • You do not need to create a function key function
  • Omit return keyword
  • Function body braces can be omitted, and represents a function that returns the contents of the body
  • No internal arguments
  • Inside this outer scope of this point
  • Not as a constructor

4、Promise

When the asynchronous request is increased, the area will become a callback, Promise asynchronous programming is a solution to solve the troubles of the developer asynchronous callback.

A Promise object has three states, and the state once changed, can no longer be changed to a different state.

• pending, asynchronous tasks in progress.
• resolved (also known as fulfilled), asynchronous tasks successfully.
• rejected, asynchronous task execution failed.

5, template string

Template string, string concatenation optimization, the ES5 by backslash 'do multiline string, for ES6 backticks' '' get directly.

// bad
const foo = 'this is a' + example;

// good
const foo = `this is a ${example}`;

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6, extended operator ?????

7, category (class)

Includes () function is used to determine whether the array contains a value of a specified return true if included, otherwise returns false.

8、for···of和for···in

for ··· of the array used to traverse

for ··· in is used to traverse the object

Reproduced in: https: //juejin.im/post/5d01a6315188251c064826a2