range.js

function* forwardRange({start, end = Infinity, step}) {
  for (let i = start; i <= end; i += step)
    yield i
}

function* reverseRange({start, end = -Infinity, step}) {
  for (let i = start; i >= end; i += step)
    yield i
}

/* eslint complexity: ['error', 7] */

/**
```
import {range} from 'async_iter/range' # conventional version
```
Returns an iterator that iterates from start to end (inclusive) by step amounts
 * @param  {Object} [opts] the optional start, end and step values for the generation
 * @param  {Object} [opts.start=0] the starting number for the iteration
 * @param  {Object} [opts.end=Number.POSITIVE_INFINITY] the end number for the iteration
 * @param  {Object} [opts.step=1] the increment for the iteration
 * @return {Iterable} An iteration for the specified range
 * @function
 * @memberof module:Generators
 * @name range
 */
export function range(opts) {
  if (typeof opts === 'number')
    opts = {end: opts}

  opts = {start: 0, ...opts}

  if (!opts.end && !opts.step)
    opts.end = Infinity

  if (!opts.step)
    opts.step = opts.start < opts.end ? 1 : -1

  return opts.step > 0 ? forwardRange(opts) : reverseRange(opts)
}