Generators

Generators are functions which can be exited and later re-entered. Useful for long iteration functions, so they can be paused to prevent blocking other functions for too long.

Basic Syntax

  1. function* myGen() { }
  2. // or 
  3. function *myGen() { }

Yield

  1. function *three() {
  2.   yield 1;
  3.   yield 2;
  4.   return 3;
  5. }
  7. var geni = three(); // starts the generator but doesn't run it
  8. geni.next(); // runs the function for one iteration. Returns { value: 1, done: false }
  9. geni.next(); // Returns { value: 2, done: false }
  10. geni.next(); // Returns { value: 3, done: true }. This ends the generator. 
  11. geni.next(); // Returns { value: undefined, done: true }

Iterating on Generators

It iterates while done = false.

  1. function *foo() {
  2.   yield 1;
  3.   yield 2;
  4.   yield 3;
  5.   yield 4;
  6.   yield 5;
  7.   return 6;
  8. }
  10. for (var v of foo()) {
  11.   console.log(v);
  12. }
  13. // Logs 1, 2, 3, 4, 5

Generator with arguments

  1. function *foo(x) {
  2.   var y = 2 * (yield (x + 1)); 
  3.   var z = yield (y / 3);
  4.   return (x + y + z); // 5 + 24 + 13
  5. }
  7. var genit = foo(5);
  9. console.log(genit.next()); // { value: 6, done: false }
  10. console.log(genit.next(12)); // { value: 8, done: false }
  11. console.log(genit.next(13)); // { value: 42, done: true }