simple-generator.js

const assert = require('assert')

function* createIncrementingSequence(increment) {
    let nextValue = 0
    for (let i = 0; i < maxIterations; i++) {
      // ↙ 'yield' each value in the iteration.
      yield nextValue
      nextValue += increment
    }
  }

  // ---------------

  // invoking a generator function creates an iterator
  let incrementByTwo = createIncrementingSequence(2)
  let sum = 0
  for (let n of incrementByTwo) {
    sum += n
  }
  assert.equal(sum, 20)

  // We can invoke the iterator manually.
  //   (You rarely need this. Just understand that there's no magic here.)
  //   (Also note that it's the same contract as the verbose iterable above.)
  incrementByTwo = createIncrementingSequence(2)
  assert.deepEqual(incrementByTwo.next(), { done: false, value: 0 })
  assert.deepEqual(incrementByTwo.next(), { done: false, value: 2 })
  assert.deepEqual(incrementByTwo.next(), { done: false, value: 4 })
  assert.deepEqual(incrementByTwo.next(), { done: false, value: 6 })
  assert.deepEqual(incrementByTwo.next(), { done: false, value: 8 })
  assert.deepEqual(incrementByTwo.next(), { done: true, value: undefined })