Generators are part of the ECMAScript 6 language specification. Firefox has supported the ECMAScript 6 compliant syntax for generators since 2013-12-10.^[1] Node.js (server-side JavaScript) also has supported ES6 generator syntax^[2] since 2013-05-15.^{[citation needed]}

example

function* vehicle(){
  var index = 0;
  while (index < 4)
    yield index++;
}

var firetruck = vehicle();
var car = vehicle();

console.log(car.next().value); // 0
console.log(car.next().value); // 1
console.log(firetruck.next().value); // 0
console.log(car.next().value); // 2
console.log(firetruck.next().value); // 1
console.log(car.next().value); // 3
console.log(car.next().value); // undefined

The function* keyword creates a generator function.^[3]

Further reading

(FIXME: Say a few more words about generators. Perhaps use some or all of the following as references:

"Generators in Node.js: Common Misconceptions and Three Good Use Cases". "Coroutines no, generators yes. Why coroutines won’t work on the Web". "What Is This Thing Called Generators?". "callbacks vs generators vs coroutines".)

References