ES6-Iterator遍历器

什么是遍历器

• 接口，为各种数据结构提供统一访问机制
• 默认具有遍历器的数据结构：数组，Map，Set，Generator函数
• 访问方式：for...of
• 一种数据结构，只要定义了Symbol.iterator属性，则实现了遍历器Interface
• Symbol.iterator可以返回next(), return(), throw()

set 数据结构Iterator使用实例

let sports = new Set().add('Basketball').add('Football').add('Tennis');

console.log(typeof sports[Symbol.iterator]);

console.log(sports[Symbol.iterator]());

for (let sport of sports) {
  console.log(sport);
}

为obj添加Iterator

function Respchain(value) {
  this.value = value;
  this.next = null;
}

Respchain.prototype[Symbol.iterator] = function() {
  var iterator = { next: next };
  var current = this;
  function next() {
    if (current) {
      var value = current.value;
      current = current.next;
      return { done: false, value: value };
    } else {
      return { done: true };
    }
  }
  return iterator;
}

let step1 = new Respchain(1);
let step2 = new Respchain(2);
let step3 = new Respchain(3);

step1.next = step2;
step2.next = step3;

for(let step of step1){
  console.log(step); 
}

区别for...in

var arr = ['a', 'b', 'c', 'd'];

for (let a in arr) {
  console.log(a); // 0 1 2 3
}

for (let a of arr) {
  console.log(a); // a b c d
}

遍历器的return()

function readLinesSync(file) {
  return {
    [Symbol.iterator]() {
      return {
        next() {
          return { done: false };
        },
        return() {
          file.close();
          return { done: true };
        }
      };
    },
  };
}