Iterators and generators

iterator

One 、for in

for…in The values returned by the loop are the key value name of the data structure .

The of the object returned by traversing the object key value ;

The subscript of the array returned by traversing the array (key).

for…in A loop can not only traverse numeric keys , It will also traverse the prototype key Values and

Other keys added manually ;

In special cases ,for…in The loop will traverse the key names in any order

forin Loops are especially good for traversing objects .

Two 、for of

forof Loop is used to get the value in a pair of key value pairs , and forin Get the key name

A data structure only needs to be deployed Symbol.iterator attribute , Is considered to have

iterator Interface , You can use forof loop .

example 3 This object , No, Symbol.iterator This attribute , So use forof Meeting

newspaper jsonisnotiterable

forof Different from forEach, It can work with break、continue and return with

Joint use , in other words forof The loop can exit the loop at any time .

It provides a unified interface for traversing all data structures ;

// Use of Traversal string 
letstr="helloworld";
for(letsofstr){
console.log(s)
}
// Use of Traversal array 
letarr=["hello","world"]
for(letkofarr.keys()){
console.log(k);
}
for(letvofarr.values()){
console.log(v);
}
for(let[k,v]ofarr.entries()){
console.log(k,v);
}
// Use in Traversing objects 
letobj={"name":"qiku","addr":" East Third Street "}
for(letkinobj){
console.log(k,obj[k]);
}

3、 ... and 、 iterator

1、 iterator Must be able to for of iteration
2、 Iteratable object Is not necessarily iterator

  Four 、 Iterative data structures

Here are the iteratable values :

•Array

•String

•Map

•Set

•Dom Elements

Use for...of Loop iterates over the data structure

Ordinary objects are created by object Created , Don't iterate

generator  

One 、 Define generator

among * Used to indicate that the function is Generator function ,yield To define a function

Internal state

generator It must be iterator

        function * myGen(){
				yield 0;
				yield 1;
				yield 2;
			}
			var x=myGen();
			for(let i of x){
				console.log(i);
			}
			console.log(x.next());
			console.log(x.next());
			console.log(x.next());
			console.log(x.next());
			console.log(x);

1、 First call next When the method is used , from Generator The head of the function begins to execute

That's ok , First it's printed one, Execute to yield Just stop , And will yield Back

42/76 Value of expression '1', As a return object value Property value , At this time, the function has not

Have you finished , Return object's done The property value is false.

2、 Second call next When the method is used , Same as the previous step .

3、 Third call next When the method is used , First it's printed three, Then the function is executed

Return operation , And will return The value of the following expression , As a return object

value Property value , At this point, the function has ended , More than done The property value is

true.

4、 The fourth call next When the method is used , At this point, the function has been executed , So back

value The property value is undefined,done The property value is true. If you execute

In the third step , No, return In words , Go straight back {value:

undefined,done:true}.

Two 、 Using the generator by object Provide Iterative interface

var obj={
                name:' Zhang San ',
                age:13,
                sex:' male ',
                hobby:' football , Volleyball , Basketball '
            }
            function * objectGen(obj){
                for(let i in obj){
                    yield [i,obj[i]];
                }
            }
            for(let i of objectGen(obj)){
                console.log(i);
            }

3、 ... and 、  Generated by big data An optimization method
    Generate data on demand

        function* bigData() {
				for (let i = 0; i < 100000; i++) {
					yield parseInt(Math.random() * 100000));
			}
			}
			var x = bigData();
			var arr = [];
			for(let i of x){
				arr.push(i);
			}
			//console.log(arr)

Four 、  Using the generator Realization Asynchronous program execution

        function* sing() {
				for (let i = 0; i < 10; i++) {
					yield ' Sing a song ' + i;
				}
			}

			function* dance() {
				for (let i = 0; i < 10; i++) {
					yield ' dance ' + i;
				}
			}

			function* rap(num) {
				for (let i = 0; i < num; i++) {
					yield 'rap' + i;
				}
			}
			var s = sing();
			var d = dance();
			var r = rap(20);
			while (true) {
				let t1 = s.next();
				console.log(t1.value);
				let t2 = d.next();
				console.log(t2.value);
				let t3 = r.next();
				console.log(t3.value);
				if (t1.done && t2.done && t3.done) {
					console.log(' complete ');
					break;
				}
			}