[Functional Programming] Create Reusable Functions with Partial Application in JavaScript

This lesson teaches you how arguments passed to a curried function allow us to store data in closure to be reused in our programs and applications. Since each argument, except for the final one, returns a new function, we can easily create reusable functions by supplying some of these arguments beforehand, and sharing these partially applied functions with other parts of our codebase. In this lesson, we'll create a curried function to fetch requests from an API that uses partial application to create reusable functionality.

Another way to do partially API call is using Proxy. Check this out.

// Partial Application

// Curried functions create a wonderful emergent property, "partial application",
// that is useful for building up reusability in our applications that you
// just can't get with normal, multivariate functions. Because curried functions
// return a new function with each argument (except for the final one), we are
// able to "partially apply" values and store them in closure, available to any
// subsequent function, thus creating new, reusable functions with some values
// already supplied.

// Imagine we have an application that needs to make requests to different APIs.
// We can create a function that bakes in the base URL, while allowing other
// arguments to be passed in later

const fetch = require('node-fetch')

const getFromAPI = baseURL => endPoint => callback =>
  fetch(`${baseURL}${endPoint}`)
    .then(res => res.json())
    .then(data => callback(data))
    .catch(err => {
      console.error(err.message)
    })

// Now we can partially apply a baseURL to create a reusable function for
// one of our APIs

const getGithub = getFromAPI(
  'https://api.github.com'
)

// We can create several get request functions by partially applying different
// endpoints to our getGithub function

const getGithubUsers = getGithub('/users')
const getGithubRepos = getGithub('/repositories')

// Now we can use our callback to get the data and do something with it.

getGithubUsers(data =>
  data.forEach(user => {
    console.log(`User: ${user.login}`)
  })
)
getGithubRepos(data =>
  data.forEach(repo => {
    console.log(`Repo: ${repo.name}`)
  })
)

// We can still continue to reuse previous partially applied functions

const getGithubOrgs = getGithub('/organizations')
getGithubOrgs(data =>
  data.forEach(org => {
    console.log(`Org: ${org.login}`)
  })
)

// We can start the process all over by partially applying a new baseURL

const getReddit = getFromAPI('https://reddit.com')

// And let's get some pictures of some cute animals

const getRedditAww = getReddit('/r/aww.json')

// And fetch the URLs of those images

const imageURLs = getRedditAww(payload =>
  payload.data.children.forEach(child => {
    console.log(
      child.data.preview.images[].source.url
    )
  })
)