findIndex

findIndex :: (Function predicate, Collection collection) => Any index

description

Finds an item's index in a Collection using a predicate function. The predicate function has the following signature: predicate :: (Any value, Any index, Collection collection) => Boolean. It should return a Boolean is called with the following arguments:

• value: the item's value

• index: the item's index or key. For arrays, the index will be a Number, for objects and maps it will be a String, and for sets, it will be equal to the item's value

• collection: the collection being iterated on.

If no item matching the predicate is found in the collection, findIndex will return null.

The predicate function can be asynchronous, which will turn the result into a Promise, requiring you to use await (or Promise.prototype.then).

examples

basic example

import { findIndex } from 'conductor'

const words = ['hello', 'world']
const predicate = word => word === 'world'
findIndex(predicate, words) // 1

searching in other data structures

import { findIndex } from 'conductor'

const words = new Set(['hello', 'world'])
const translator = { hola: 'hello', mundo: 'world' }
const otherTranslator = new Map([['bonjour', 'hello'], ['monde'], ['world']])
const predicate = word => word === 'world'

findIndex(predicate, words) // 'world'
findIndex(predicate, translator) // 'mundo'
findIndex(predicate, otherTranslator) // 'monde'

We're using findIndex on data structures which are not arrays, and we are still looking for the index of the word 'world'. Here, words is a Set, so the associated index is equal to the value, which is world. translator is an Object, and the key associated to the 'world' is the string 'mundo'. otherTranslator is a Set, translating French to English, and the key associated to 'world' is 'monde'.

using an asynchronous predicate

import { findIndex } from 'conductor'

const character_ids = [1, 3]
const getCharacter = id => fetch(`https://swapi.co/api/people/${id}`).then(res => res.json())
const predicate = id => getCharacter(id).then(character => character.name === 'R2-D2')
await findIndex(predicate, character_ids) // 1

Here, we have an asynchronous predicate which takes a character_id and checks if the character is R2-D2, using the Star Wars API. Since the predicate is asynchronous, we need to use await because the result is a Promise. Luckily, we found the droid we were looking for 😎.