Iterators and generators

1. iterator

        iterator API Use next() Method traverses data in an iteratable object , Object etc. ,next() Method returns the iterator object IteratorResult Contains two properties ： done and value.

let arr = ['foo'];
let iter = arr[Symbol.iterator]();
console.log(iter.next()); // { done: false, value: 'foo' }
console.log(iter.next()); // { done: true, value: undefined }
console.log(iter.next()); // { done: true, value: undefined }
console.log(iter.next()); // { done: true, value: undefined }

To understand the principle , You can customize an iterator

// Define a class 
class Counter {
// Counter  The instance of should iterate  limit  Time 
constructor(limit) {
this.count = 1;
this.limit = limit;
}

next() {
if (this.count <= this.limit) {
return { done: false, value: this.count++ };
} else {
return { done: true, value: undefined };
}
}
[Symbol.iterator]() {
return this;
}
}

// Instantiate the class as an object 
let counter = new Counter(3);
for (let i of counter) {
console.log(i);
}
// 1
// 2
// 3

2. generator （ Arrow functions cannot be used to define generator functions ）

Generators are used to add elements to iterators , use yield Keyword to add an element to an iterator
generator

    function *createIterator(items) {
        for(i=0;i<items.length;i++){
            yield items[i]
        }
    }
    arr = [1,2,3]
    it = createIterator(arr)
    console.log(it.next())
    console.log(it.next())
    console.log(it.next())