What are ES6 generators and how can I use them in node.js?

Question

I was at a node.js meetup today, and someone I met there said that node.js has es6 generators. He said that this is a huge improvement over callback style programming, and w

Answer 1

Summary:

function* defines a generator function which returns a generator object. The special thing about a generator function is that it doesn't execute when it is called using the () operator. Instead an iterator object is returned.

This iterator contains a next() method. The next() method of the iterator returns an object which contains a value property which contains the yielded value. The second property of the object returned by yield is the done property which is a boolean (which should return true if the generator function is done).

Example:

function* IDgenerator() {
  var index = 0;

  yield index++;
  yield index++;
  yield index++;
  yield index++;
    
}

var gen = IDgenerator(); // generates an iterator object

console.log(gen.next().value); // 0  
console.log(gen.next().value); // 1
console.log(gen.next().value); // 2
console.log(gen.next()); // object, 
console.log(gen.next()); // object done

In this example we first generate an iterator object. On this iterator object we then can call the next() method which allows us to jump form yield to yield value. We are returned an object which has both a value and a done property.

How is this useful?

• Some libraries and frameworks might use this construct to wait for the completion of asynchronous code for example redux-saga
• async await the new syntax which lets you wait for async events uses this under the hood. Knowing how generators work will give you a better understanding of how this construct works.