filter

filter :: (Function predicate, Collection collection) => Collection | Promise<Collection>

description

Iterates over a collection (Array, Object, Map, Set) and returns a new collection of the same type containing only the values for which the predicate function evaluates to true.

Like many Collection methods in Conductor, filter works with both asynchronous & synchronous mappers. If you use a synchronous predicate function, filter will work like Array.prototype.filter and return a Collection synchronously.

const values = [0, 1, 2, 3]
const isEven = x => x % 2 === 0
filter(isEven, values) // [0, 2]

If you use an asynchronous mapper, filter will return a Promise, and you will need to use await or Promise.prototype.then to retrieve the new collection.

const values = [0, 1, 2, 3]
const isEven = async x => x % 2 === 0
filter(double, values) // Promise<Pending>
await filter(double, values) // [0, 2]

If you use an asynchronous predicate function, all calls to the predicate function will be done in parallel, but the input collection's order will be preserved.

examples

basic example

import { filter } from 'conductor'

const values = [0, 1, 2, 3]
const isEven = x => x % 2 === 0
filter(isEven, values) // [0, 2]

Here, we are just using filtering on an array to keep only even values. The predicate function is synchronous.

other data structures

import { filter } from 'conductor'

const object = { drummer: 1, drumsticks: 2 }
const map = new Map([['drummer', 1], ['drumsticks', 2]])
const set = new Set([0, 1, 2, 3])
const isEven = x => x % 2 === 0
filter(isEven, object) // { drumsticks: 2 }
filter(isEven, map) // Map { 'drumsticks' => 2 }
filter(isEven, set) // Set { 0, 2 }

using the index in the predicate function

import { filter } from 'conductor'

const words = ['hello', 'hello', 'world', 'world']
const isEven = x => x % 2 === 0
filter((_, index) => isEven(index), words) // ['hello', 'world']

using an asynchronous predicate

import { filter } from 'conductor'

const values = [0, 1, 2, 3]
const isEven = async x => x % 2 === 0
await filter(double, values) // [0, 2]

If your predicate function is asynchronous, you will need to use await (or Promise.prototype.then) because the result will be a Promise.