JS--ES 2015/6 new features summary

ES 2015/6 has quite a lot of new content. Here is only an outline (not necessarily comprehensive) list of these features. In fact, there will be a lot of knowledge in every point you dig into. This article aims to summarize it, so it will not conduct in-depth discussion and research on these characteristics. Then, if I have time, I will write a few separate blogs to dig deeper into commonly used points and have in-depth exchanges with everyone.

Arrow function

Arrow function is a short form of function implemented through => syntax. Similar syntax is available in C#/JAVA8/CoffeeScript. Unlike functions, arrow functions share the same this with their execution context. If an arrow function appears inside a function object, it shares arguments variables with the function.

// Expression bodiesvar odds = evens.map(v => v + 1);var nums = evens.map((v, i) => v + i);// Statement bodiesnums.forEach(v => {
  if (v % 5 === 0)    fives.push(v);});// Lexical thisvar bob = {
  _name: "Bob",
  _friends: ['jim'],
  printFriends() {
    this._friends.forEach(f =>
      console.log(this._name + " knows " + f)); // Bob knows jim
  }};// Lexical argumentsfunction square() {
  let example = () => {
    let numbers = [];
    for (let number of arguments) {
      numbers.push(number * number);
    }

    return numbers;
  };

  return example();}square(2, 4, 7.5, 8, 11.5, 21); // returns: [4, 16, 56.25, 64, 132.25, 441]

Class Class

Javascript Class does not introduce a new object-oriented approach The object inheritance model is based on syntactic sugar of prototypal inheritance. It provides a simpler and clearer syntax for creating objects and handling inheritance.

class Rectangle {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }}

The class does not declare promotion, you must ensure that it has been declared before calling.

Constructor constructor is a special method used to create and initialize instances of a class.

Static methodstatic The keyword is used to declare a static method

Create a subclassextends The keyword is used to create a subclass, please note here: extends cannot be used to extend regular objects (non-constructible/non-constructible). If you want to inherit regular objects, you can use Object.setPrototypeOf().

Call super classsuper Keyword can be used to call methods in the parent class

Mix-ins Mix

Enhancement The object literal

can be implemented in the literal form to define prototype, key-value pair abbreviation, definition method, etc., dynamic attribute names.

var obj = {
    // Sets the prototype. "__proto__" or '__proto__' would also work.
    __proto__: theProtoObj,
    // Computed property name does not set prototype or trigger early error for
    // duplicate __proto__ properties.
    ['__proto__']: somethingElse,
    // Shorthand for ‘handler: handler’
    handler,
    // Methods
    toString() {
     // Super calls
     return "d " + super.toString();
    },
    // Computed (dynamic) property names
    [ "prop_" + (() => 42)() ]: 42};

Template string

Template string provides syntactic sugar for constructing strings, in Prel/python, etc. Languages ​​also have similar features.

// Basic literal string creation
`This is a pretty little template string.`

// Multiline strings
`In ES5 this is
 not legal.`

// Interpolate variable bindings
var name = "Bob", time = "today";
`Hello ${name}, how are you ${time}?`

// Unescaped template strings
String.raw`In ES5 "\n" is a line-feed.`

// Construct an HTTP request prefix is used to interpret the replacements and construction
GET`http://foo.org/bar?a=${a}&b=${b}
    Content-Type: application/json
    X-Credentials: ${credentials}
    { "foo": ${foo},
      "bar": ${bar}}`(myOnReadyStateChangeHandler);

Destructuring assignment

The Destructuring method is a Javascript expression, which makes it possible to extract a value from an array or a property from an object into a different variable.

// list matching
var [a, ,b] = [1,2,3];
a === 1;
b === 3;

// object matching (用新变量名赋值)
var { op: a, lhs: { op: b }, rhs: c }
       = getASTNode()

// object matching shorthand
// binds `op`, `lhs` and `rhs` in scope
var {op, lhs, rhs} = getASTNode()

// Can be used in parameter position
function g({name: x}) {
  console.log(x);
}
g({name: 5})

// Fail-soft destructuring
var [a] = [];
a === undefined;

// Fail-soft destructuring with defaults
var [a = 1] = [];
a === 1;

// 变量可以先赋予默认值。当要提取的对象没有对应的属性，变量就被赋予默认值。
var {a = 10, b = 5} = {a: 3};
console.log(a); // 3
console.log(b); // 5

// Destructuring + defaults arguments
function r({x, y, w = 10, h = 10}) {
  return x + y + w + h;
}
r({x:1, y:2}) === 23

// 对象属性计算名和解构
let key = "z";
let { [key]: foo } = { z: "bar" };

console.log(foo); // "bar"

Default + Rest + Spread

Provide default values ​​for function parameters& ... Fixed number of parameters

function f(x, y=12) {
  // y is 12 if not passed (or passed as undefined)
  return x + y;
}
f(3) == 15


function f(x, ...y) {
  // y is an Array
  return x * y.length;
}
f(3, "hello", true) == 6



function f(x, y, z) {
  return x + y + z;
}
// Pass each elem of array as argument
f(...[1,2,3]) == 6

Let + Const

let is used to declare block-level scope variables. const is used to declare constants.

function f() {
  {
    let x;
    {
      // this is ok since it's a block scoped name
      const x = "sneaky";
      // error, was just defined with `const` above
      x = "foo";
    }
    // this is ok since it was declared with `let`
    x = "bar";
    // error, already declared above in this block
    let x = "inner";
  }
}

Iterator

Custom iterators can be created through symbol.iterator.

let fibonacci = {
  [Symbol.iterator]() {
    let pre = 0, cur = 1;
    return {
      next() {
        [pre, cur] = [cur, pre + cur];
        return { done: false, value: cur }
      }
    }
  }
}

for (var n of fibonacci) {
  // truncate the sequence at 1000
  if (n > 1000)
    break;
  console.log(n);
}

Generators

Normal functions use function declarations, while generator functions use function* declarations.

Inside the generator function, there is a syntax similar to return: the keyword yield. The difference between the two is that a normal function can only return once, while a generator function can yield multiple times (of course it can also yield only once). During the execution of the generator, it will pause immediately when encountering a yield expression, and the execution state can be resumed later.

function* quips(name) {
  yield "你好 " + name + "!";
  yield "希望你能喜欢这篇介绍ES6的译文";
  if (name.startsWith("X")) {
    yield "你的名字 " + name + "  首字母是X，这很酷！";
  }
  yield "我们下次再见！";
}

Unicode

// same as ES5.1
"

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