JS Generator Extension

Array-like methods for generators.

Background

In C# and Python, we can do some operations on an iterator and get chained generators.

(x * 10 for x in range(10) if x % 2 == 1)
# got a generator for [10, 30, 50, 70, 90]

Enumerable.Range(0, 10)
    .Where(x => x % 2 == 1)
    .Select(x => x * 10)
// got a iterator for [10, 30, 50, 70, 90]
// using LINQ extension methods

from x in Enumerable.Range(0, 10)
    where x % 2 == 1
    select x * 10
// the same, using LINQ query syntax

Currently, to do the same thing in JavaScript:

new Array(10).fill(0).map((x, i) => i)
  .filter(x => x % 2 == 1)
  .map(x => x * 10)

There are a few disadvantages:

• new Array(n).fill(...).map(...) is too long and confusing
• 3 intermediate arrays are created (new Array(), the first map(), filter())

Getting Started

npm install generator-ext

import { range } from "generator-ext";

[...range(10).filter(x => x % 2 == 1).map(x => x * 10)]

• We have range() to get a Python-like range generator
• We can call methods like map() on any generator, retuning chained generator
• They are just (chained) generators. We can call toArray() on the generator to get the result as array, without any intermediate arrays.
  • [...generator] also works.

Methods for Generators

These methods are added to the prototype of GeneratorFunction:

• Array-like methods (map, filter, flat, flatMap, reduce)

  • are identical to Array builtins, excepts they return generators instead of arrays.

• toArray method

range Generator Function

Generates a number sequences. It's similar to range in Python.

range(10)
// generator for [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

range(3, 10)
// generator for [3, 4, 5, 6, 7, 8, 9]

range(3, 10, 2)
// generator for [3, 5, 7, 9]