lazy-arr 
Arrays that look just like regular JavaScript arrays, but are computed lazily. Like Scala or Haskell's lazy streams. Read more about it in the introductory blog post: https://performancejs.com/post/ewffd34/Introducing:-Lazy-arrays-in-JavaScript.
Install
npm i lazy-arr -SUsage
Lazy-arr takes a function, and uses it to lazily generate values for the array. The function takes a numerical array index (eg. 5) and should return the value for that index in the array. The function doesn't have to be idempotent, but its return value will be cached (you can then delete it from cache, if you want).
It supports 2 usage patterns:
- Call it with just a function:
import { lazy } from 'lazy-arr'
lazy(index => index + 1)- Call it with a function and an initial value:
import { lazy } from 'lazy-arr'
let seq = lazy([0])(index => index + seq[index - 1])Examples
import { lazy } from 'lazy-arr'
// even numbers
let numbers = lazy(_ => _ * 2)
numbers[0] // 0
numbers[5] // 10
// fibonacci numbers (with initial value of [0, 1])
let fibs = lazy([0, 1])(_ => fibs[_ - 1] + fibs[_ - 2])
fibs[0] // 0
fibs[1] // 1
fibs[10] // 55Other operations
import { lazy } from 'lazy-arr'
let numbers = lazy(_ => _ * 2)
numbers[3] // 6
// membership
2 in numbers // true
3 in numbers // true
4 in numbers // false
// deleting
delete numbers[3]
3 in numbers // falseNote that you cannot directly set values:
import { lazy } from 'lazy-arr'
let numbers = lazy(_ => _ * 2)
numbers[7] = 3 // THROWS ERRORLicense
MIT