ES6 — generators
Let’s talk about ECMAScript 2015
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.
Like it? Recommend it!
Whole series is also available as an ebook. I published it on leanpub.com
Future is bright!
