Creating and Iterating through a Generator Object

Iterating through a Generator Object

Creating a Generator Object

A Generator Object is returned from calling a Generator function. It is important to not confuse Generator Objects with Generator functions.

Notice how a Generator Object is created by calling a Generator function:

function* genFunc() {
    console.log("started");
    yield 'a';
    console.log("passed first yield");
    yield;
    console.log("passed second yield");
    yield 123;
    console.log("passed third yield");

    return "finished";
}

let genObject = genFunc(); //creates a generator object called genObject

Iterating through a Generator Object with next()

Generator Objects conform to the iterator protocol and may be iterated with the next() method.

Generator functions are initially paused and the first call to next() starts the Generator function. The Generator function then runs until it hits the first yield keyword and then pauses. Subsequent calls to next() will resume the Generator function until the next yield keyword appears.

The next() method returns an object with two properties:

• done - a boolean indicating whether the Generator function has processed all of the yield statements or has already returned.
• value - the value associated with the most recent yield statement.

Notice how the next() method is used to iterate through all of the yield statements:

let a = genObject.next(); // Object {value: 'a', done: false}
//console.log("started");

let b = genObject.next(); // Object {value: undefined, done: false}
//console.log("passed first yield"); 

let c = genObject.next(); // Object {value: 123, done: false}
//console.log("passed second yield");

After all of the yield statements have been processed with next(), the following next() call returns an object with a value property equal to the Generator function return value and a done property set to true. If the return statement was omitted from the Generator function then the value property will be undefined. After the the done property is true in one of the returned objects, additional next() calls will return objects with an undefined value property and a true done property. Yield statements after the return statement are ignored.

Notice how additional calls to next() behave:

let d = genObject.next(); // Object {value: "finished", done: true} <-- value property takes the return value of genFunc()
//console.log("passed third yield");

let e = genObject.next(); // Object {value: undefined, done: true} <-- additional next() calls return this