Create a JavaScript `iterable` object without the use of generator functions

manual-iterable.js

// Gah!
// Don’t try to understand all of this.
class IncrementingSequence {
  
  constructor(increment) {
    this.increment = increment
  }
 
  // ↙ Iteration function specially named with a “Symbol” defined by the language
  [Symbol.iterator]() {
    const maxIterations = 5
    let nextValue = 0
    let i = 0
    let self = this
    // The iteration function returns an object with a method 
    //   ‘next’. `next()` will be called repeatedly when iterating.
    return {
      next() {
      // Each step in the iteration returns an object with a ‘value’
      //   and a ‘done’ flag
      let result = {
        done: i >= maxIterations,
        value: nextValue
      }
      i++
      nextValue += self.increment
      return result
    }
  }
}

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

// We can iterate through our custom iterable with a "for-of" loop, just
//   like an array.
const assert = require('assert')

let sum = 0
let incrementByTwo = new IncrementingSequence(2)
for (let n of incrementByTwo) {
  sum += n
}
assert.equal(sum, 20)