generators

Generators are simply subtypes of Iterators which I wrote about previously. They are a special kind of function that can be suspended and resumed, which is making a difference to iterators. Generators use function* and yield operators to work their magic.

The yield operator returns a value from the function and when the generator is resumed, execution continues after the yield.

We also have to use function* (with star character) instead of a function to return a generator instance.

!!! Generators have been borrowed from Python language.

The most magical feature in ES6!

Why? Take a look:

function* generator () {
  yield 1;
  // pause
  yield 2;
  // pause
  yield 3;
  // pause
  yield 'done?';
  // done
}
let gen = generator(); // [object Generator]

console.log(gen.next()); // Object {value: 1, done: false}
console.log(gen.next()); // Object {value: 2, done: false}
console.log(gen.next()); // Object {value: 3, done: false}
console.log(gen.next()); // Object {value: 'done?', done: false}
console.log(gen.next()); // Object {value: undefined, done: true}
console.log(gen.next()); // Object {value: undefined, done: true}

for (let val of generator()) {
  console.log(val); // 1
                    // 2
                    // 3
                    // 'done?'
}

As you can see, the generator has four yield statements. Each returns a value, pauses execution and moves to the next yield when next() method is called. Calling a function produces an object for controlling generator execution, a so-called generator object.

Use is similar to iterators. We have next() method as I mentioned above and we can even use it with for-of loop.

Below is an example of a generator called random1_10, which returns random numbers from 1 to 10.

function* random1_10 () {
  while (true) {
    yield Math.floor(Math.random() * 10) + 1;
  }
}

let rand = random1_10();
console.log(rand.next());
console.log(rand.next());
// …

Generator has never ending while loop. It produces random numbers every time when you call next() method.

ES5 implementation:

function random1_10 () {
  return {
    next: function() {
      return {
        value: Math.floor(Math.random() * 10) + 1,
        done: false
      };
    }
  };
}

let rand = random1_10();
console.log(rand.next());
console.log(rand.next());
// …

We can also mix generators together:

function* random (max) {
  yield Math.floor(Math.random() * max) + 1;
}

function* random1_20 () {
  while (true) {
    yield* random(20);
  }
}

let rand = random1_20();
console.log(rand.next());
console.log(rand.next());
// …

random1_20 generator returns random values from 1 to 20. It uses random generator inside to create random number each time when yield statement is reached.