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LazyIterable

A lightweight, powerful library that provides wrapper classes and utility functions for lazily-evaluating data structures, including support for async sources, and built completely with native JavaScript features.

Motivation

This package was motivated by my desire to learn how generators work in JavaScript, and then spiralled into learning how to setup and publich a fully featured NPM package. It was partly inspired by the pythonic package.

It is written in TypeScript, tested with Jest, linted with ESLint, formatted with Prettier, bundled with Rollup, and documented with TSDoc.

Installation

Using npm:

npm install @jderrington/lazy-iterable

Using yarn:

yarn add @jderrington/lazy-iterable

Importing

const {LazyIterable} = require("@jderrington/lazy-iterable");
// Or
import {LazyIterable} from "@jderrington/lazy-iterable";

Using script tags:

/* Take the index.js file from the dist/ folder */
<script src="index.js"></script>
<script>
/* Imports are under the 'LazyIterableLib' name */
const {LazyIterable} = LazyIterableLib;
</script>

Usage

See the documentation for full the full API specification and examples. There are also examples of usage with Node streams and pagination.

Provides Python and Haskell style functions such as enumerate, range, zip and repeat, as well as lazy methods such as take, and drop (and many more).

// Track the index in a for loop instead of requiring `forEach`
for (const [index, value] of enumerate([1, 2, 3, 4, 5])) {
  // Loop body
}

// Python style range iteration
for(const i of range(0, 5)) {
  // Loop body
}

// Haskell style lazy evaluation
repeat(5).take(5).toArray() // Returns [5, 5, 5, 5, 5]

// Use familiar array methods on infinite data sources
infinite()
  .map((value, index) => `${index}: ${value}`)
  .take(3)
  .join(", "); // Returns "0: 1, 1: 2, 2: 3"

Index

Functions

enumerate

  • enumerate<T>(iterable: Iterable<T>): LazyIterable<[number, T]>
  • Returns a LazyIterable that yields the elements of the given iterable alongside the index as an array pair.

    This method is lazy, as opposed to the built in Array.forEach() method. This means it can be chained with other methods without building intermediate arrays, or as a more powerful version of a for..of loop.

    example

    Enumerate an iterable

    for (const [index, value] of enumerate(["A", "B", "C"])) {
      console.log(`${index}: ${value}`)
    }
    
    see

    enumerateAsync for the async version

    Type parameters

    • T

    Parameters

    • iterable: Iterable<T>

      The iterable to enumerate

    Returns LazyIterable<[number, T]>

enumerateAsync

  • Async version of enumerate.

    example

    Enumerate an AsyncLazyIterable

    async function* generator() { yield* ["A", "B", "C", "D"] }
    const result = await enumerateAsync(new AsyncLazyIterable(generator())).toArray() // Returns [[0, "A"], [1, "B"], [2, "C"], [3, "D"]]
    
    see

    enumerate for the synchronous version

    Type parameters

    • T

    Parameters

    • iterable: AsyncIterable<T>

      The async iterable to enumerate

    Returns AsyncLazyIterable<[number, T]>

infinite

  • infinite(start?: number, step?: number): LazyIterable<number>
  • Returns a LazyIterable that yields an unending steam of numbers, starting at start and separated by step. Both start and step default to 1.

    example

    Generate an infinite increasing list

    infinite(0, 1).take(5).toArray() // Returns [0, 1, 2, 3, 4]
    

    Parameters

    • start: number = 1

      The first number yielded from the LazyIterable

    • step: number = 1

      The gap between sucessive yielded values

    Returns LazyIterable<number>

range

  • range(start: number, stop: number, step?: number): LazyIterable<number>
  • Returns a LazyIterable that yields values between start (inclusive) and stop (exclusive), in steps of size step (default 1). Start can be greater than stop, in which case the LazyIterable will yield decreasing values - step should still be positive in this case.

    example

    Generate ranges of numbers

    range(0, 5).toArray() // Returns [0, 1, 2, 3, 4]
    range(0, 10, 2).toArray() // Returns [0, 2, 4, 6, 8]
    range(20, 0, 5).toArray() // Returns [20, 15, 10, 5]
    

    Parameters

    • start: number

      The first value to be yielded from the LazyIterable

    • stop: number

      The bound on the last value to be yielded

    • step: number = 1

      The difference between successive yielded values

    Returns LazyIterable<number>

repeat

  • Returns a LazyIterable that yields the given value infinitely.

    example

    Generate an infinite stream of the value 1

    repeat(1).take(5).toArray() // Returns [1, 1, 1, 1, 1]
    

    Type parameters

    • T

    Parameters

    • value: T

    Returns LazyIterable<T>

zip

  • zip<A, B>(iterableA: Iterable<A>, iterableB: Iterable<B>): LazyIterable<[A, B]>
  • Returns a LazyIterable that yields pairs of values as an array, generated by iterating both provided iterables. The number of elements yielded by the resulting LazyIterable is the length of the shorter of the two iterables.

    example

    Zip two arrays together

    zip(["A", "B", "C"], [1, 2, 3]).toArray() // Returns [["A", 1], ["B", 2], ["C", 3]]
    
    see

    zipWith for a combination of zip and map.

    Type parameters

    • A

    • B

    Parameters

    • iterableA: Iterable<A>
    • iterableB: Iterable<B>

    Returns LazyIterable<[A, B]>

zipWith

  • zipWith<A, B, C>(callback: (a: A, b: B, index: number) => C, iterableA: Iterable<A>, iterableB: Iterable<B>): LazyIterable<C>
  • Returns a LazyIterable that yields values generated by zipping the two given iterables and passing the results through the provided callback; effectively, a combination of zip and map.

    example

    Combine two iterables together with a callback

    zipWith((a, b) => `${a} => ${b}`, ["A", "B", "C"], [1, 2, 3]).toArray() // Returns ["A => 1", "B => 2", "C => 3"]
    
    see

    zip for the basic method.

    Type parameters

    • A

    • B

    • C

    Parameters

    • callback: (a: A, b: B, index: number) => C
        • (a: A, b: B, index: number): C
        • Parameters

          • a: A
          • b: B
          • index: number

          Returns C

    • iterableA: Iterable<A>
    • iterableB: Iterable<B>

    Returns LazyIterable<C>

Legend

  • Method

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