with-iterator

with-iterator

A utility for attaching iterator factories / generators to arbitrary input.

• Is mutative - when passed a composite type, returns the same reference.
• Is type aware - when passed a primitive, returns the same object type.
• Is curried - when passed a single function, composes a new wrapper.

TL;DR examples

motivation

This project originally served as a simple aid in understanding the mechanics of publishing to NPM while also an excuse to play around with generator functions, which I generally don't find many use-cases for in day-to-day app development - for the same reason this package has limited application.

install

Node

> npm install with-iterator

import { withIterator } from 'with-iterator'

Deno

import { withIterator } from 'npm:with-iterator'

TypeScript

Depending on existing environment or project setup the following minimal config may also be required. strictNullChecks is optional but will improve type resolution if enabled.

{
    "compilerOptions": {
        "strictNullChecks": true,
        "lib": ["es5", "es2015.iterable", "es2015.generator"]
    }
}

exposes

Note: The documented types here are simplified for readability. Inner types should be preserved - see source for detail.

withIterator

function withIterator(
    factory: () => Generator,
    input: any,
    descriptor?: PropertyDescriptor,
): Iterable

function withIterator(
    factory: () => Generator,
): (input: any, descriptor?: PropertyDescriptor) => Iterable

function withIterator(input: any): Iterable

The optional descriptor parameter corresponds to that of Object.defineProperty and if not overridden, is set with sensible defaults:

{
    writable: true,
    configurable: true,
    enumerable: false
}

isIterable

function isIterable(input: any): boolean

getIterator

function getIterator(input: any): unknown | () => Generator

valueOf

function valueOf(input: any): any

examples

» StackBlitz examples

usage

import { withIterator, isIterable, getIterator, valueOf } from 'with-iterator'

By default if not passed a factory function, any input is assigned an iterator that yields itself, if it isn't already iterable - else it is unchanged.

Array.from(withIterator([1, 2, 3])) // [ 1, 2, 3 ]
Array.from(withIterator({ value: true })) // [ { value: true } ]

When explicitly passed a factory, any existing iterator is superseded.

const sum = function* allThatCameBefore() {
    let i = 0
    let n = 0
    while (i < this.length) {
        yield (n += this[i])
        i++
    }
}
const foo = withIterator(sum, [1, 2, 3, 4, 5])
Array.from(foo) // [ 1, 3, 6, 10, 15 ]

When only passed a factory, will compose a new helper. This examples also demonstrates the use of primitive input.

const hex = ['123456', 'cc9933', 'fedcba']
const rgb = withIterator(function* () {
    const re = /../g
    let next
    while ((next = re.exec(this))) yield Number.parseInt(next, 16)
})
hex.map(rgb).forEach((code) => {
    console.log(`hex: ${code} - rgb: (${[...code]})`)
})
// hex: 123456 - rgb: (18,52,86)
// hex: cc9933 - rgb: (204,153,51)
// hex: fedcba - rgb: (254,220,186)

Assigning an iterator to a class prototype.

class Digits extends Number {}
withIterator(function* () {
    for (let d of getIterator(String).call(this)) yield Number(d)
}, Digits.prototype)

const digits = new Digits(54321)
Array.from(digits) // [5, 4, 3, 2, 1]

Resolving boxed primitives.

const nul = withIterator(null)
const und = withIterator(undefined)
const num = withIterator(42)
console.log(
    valueOf(nul), // null
    valueOf(und), // undefined
    valueOf(num), // 42
)