JS interview in 2 minutes / Higher Order Functions

#javascript #typescript #beginners #career

Question:
Explain Higher Order Functions in Javascript.

Quick answer:
These are functions that return other functions.

UPD: as @mazentouati noted in the comments, Higher Order Functions are also functions that accept a function as its parameter. Wikipedia

Longer answer:
In JavaScript, you can return objects of any type as a result of the function or receive an object of any type as a parameter. This means you can create a function that will return a function.

function higherFunction() {
  return function regularFunction() {
    console.log('Hello world!')
  }
}

const a = higherFunction() // won't print anything
a() // Hello world!

You can also try to create even more nested functions.

const a = () => () => () => () => console.log('Hello world')
const b = a()
const c = b()
const d = c()

d() // Hello world!

You can pass functions to a function that will execute functions in some specific order.

const pipe = (...args) => 
  (init) => 
    args.reduce((acc, cur) => cur(acc), init)

const a = pipe(
 (val) => val + 1,
 (val) => val * 2,
 (val) => console.log("Got", val),
)

a(10) // Got 22

And more other fun ways to use functions 🤪

Real-life example:
Some frameworks (angular) and libraries (MobX) heavily rely on decorators, but decorators are no more than Higher Order Functions themselves.

const logDecorator = (wrapped) => {
  return (...args) => {
    console.log(`Invoked ${wrapped.name} with args ${args}`)
    wrapped(...args)
  }
}

const tmp = (param) => {
  console.log('Do nothing, but just show param:', param) 
}

const wrappedTmp = logDecorator(tmp)
wrappedTmp('Hello world!')
// Invoked tmp with args Hello world!
// Do nothing, but just show param: Hello world!

Some other libraries (RxJs) may use it as configurable helpers.

// These functions can by provided by a library itself
const uppercase = (a) => a.toUpperCase();
const removePunctuation = (a) => a.replace(/[^0-9a-zA-Z ]/g, '')

// pipe is a Higher Order Function that returns a function, which will apply all functions one by one
const process = pipe(
  uppercase,
  removePunctuation,
)

console.log(process('qwe-qwe'), process('Hello world!'))
// QWEQWE HELLO WORLD