Yielding to other Generators

Yield* Keyword

The yield* keyword is used to call another Generator function within a Generator function.

Notice how the yield* statement is used to call genFuncA() within genFuncB():

function* genFuncA() {
    yield 'a';
    yield 'b';
    yield 'c';

    return "done with genFuncA()!"

}

function* genFuncB(){
    yield 1;
    yield* genFuncA(); // contains iterable [a,b,c]
    yield 2;
    yield 3;

    return "done with genFuncB()!";
}

let genObject = genFuncB();

let a = genObject.next(); //Object {value: 1, done: false}
let b = genObject.next(); //Object {value: 'a', done: false}
let c = genObject.next(); //Object {value: 'b', done: false}
let d = genObject.next(); //Object {value: 'c', done: false}
let e = genObject.next(); //Object {value: 2, done: false}
let f = genObject.next(); //Object {value: 3, done: false}
let g = genObject.next(); //Object {value: "done with genFuncB()!", done: true}

The yield* statement does not add the return value of the generator function that it calls to its list of iterables. Instead, the return value may be accessed by the return value of the yield* statement.

Notice how the yield* genFuncA() statement returns the return value of genFuncA():

function* genFuncA() {
    yield 'a';
    yield 'b';

    return "done with genFuncA()!"

}

function* genFuncB(){
    yield 1;
    var returnVal = yield* genFuncA(); // contains iterable list [a,b] and returns with value "done with genFuncA()!"
    yield returnVal; // returnVal is equal to"done with genFuncA()
    yield 2;

    return "done with genFuncB()!";
}

var genObject = genFuncB();

var a = genObject.next(); //Object {value: 1, done: false}
var b = genObject.next(); //Object {value: 'a', done: false}
var c = genObject.next(); //Object {value: 'b', done: false}
var d = genObject.next(); //Object {value: "done with genFuncA()!", done: false}
var e = genObject.next(); //Object {value: 2, done: false}
var f = genObject.next(); //Object {value: "done with genFuncB()!", done: true}

The yield* statement can be used on any iterable in addition to Generator functions.

Notice how the yield* statement is used to yield all of the values of in an array:

function* genFunc(){
    yield 1;
    yield* [2,3,4]; //the array [2,3,4] is iterable
    yield 5;

}

let genObject = genFunc();

let a = genObject.next(); //Object {value: 1, done: false}
let b = genObject.next(); //Object {value: 2, done: false}
let c = genObject.next(); //Object {value: 3, done: false}
let d = genObject.next(); //Object {value: 4, done: false}
let e = genObject.next(); //Object {value: 5, done: false}
let f = genObject.next(); //Object {value: undefined, done: true}