Iterable Numbers in JavaScript with Symbol.iterator

Kudos to Samantha Ming for her thoughts on printing ranges natively on JavaScript.

This looked really promising and cool. So I though about implementing within a custom class and without generators so it can run without regenerator-runtime in Nodejs.

This is what I came up with and it doesn’t have any side effects mentioned by the original article.

The purpose of the class is to create a number that can be spread into an array reaching to 0 from either positive or negative side.

Since the implementation basically uses Symbol.iterator it also works for looping with for ...of.

This is what the code looks like.

/**
 * An experimental iterable number class
 *
 * It implements Symbol.iterator, so one can use it with for-of loop and spread.
 *
 * This is just an experiment, I don't know why someone would actually use it,
 * but if you are and need some sort of license, here you go.
 *
 * copyright 2018 Swashata Ghosh <swashata@wpquark.com>
 * This code is licensed under MIT license
 */
class IterableNumber {
  constructor(num = 0) {
    if (Number.isNaN(Number.parseInt(num, 10)) || !Number.isFinite(num)) {
      throw new Error('Must pass a valid finite integer');
    }
    this.num = num;
    this[Symbol.iterator] = () => ({
      next: this.generateNext(this.num),
    });
  }

  generateNext = (top) => {
    // increase the number by one, if positive, else decrease by one
    let running = top > 0 ? top + 1 : top - 1;
    return () => {
      // If the running number is zero, then it was done
      // in the previous step
      if (running === 0) {
        return {
          done: true,
        };
      }
      // Otherwise, let's decrease or increase the number, based on
      // positivity
      running = this.num > 0 ? running - 1 : running + 1;
      return {
        value: running,
        done: false,
      };
    };
  };
}

Now we can use it like this.

const spreadedNum = [...new IterableNumber(9)];
console.log(spreadedNum); // [ 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 ]

for (const i of new IterableNumber(20)) {
  console.log(i); // 20, 19, 18, ... 1, 0
}

const spreadedNumN = [...new IterableNumber(-9)];
console.log(spreadedNumN); // [ -9, -8, -7, -6, ..., -2, -1, 0 ]

const spreadedZ = [...new IterableNumber(0)];
console.log(spreadedZ); // [ 0 ]

try {
  const invalidSpread = [...new IterableNumber('duh?')];
  console.log(invalidSpread);
} catch (e) {
  console.log(e); // [Error: Must pass a valid finite integer]
}

If you have any questions, shoot.