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Functional Programming in JavaScript Cheat Sheet Cheat Sheet by

This is a cheat sheet describing the basics of FP in JS

Functional Progra­mming

Functional progra­mming is a declar­ative paradigm of building software by composing pure functions.

Pure Functions

A pure function is a function which:
1. Given the same input, returns the same output.
2. Has no side-e­ffects

Benefits of Functional Progra­mming

It helps us to solve problems effect­ively in a simpler way
It improves modularity
It reduces complex problems into simple pieces
It helps us to debug the code quickly

Arrow functions

const incr = x => x + 1
Add 2 numbers
const add = (x, y) => x + y
Square root
const squareRoot = x => Math.s­qrt(x)
const square = x => x * x


Add 2 numbers
const add = x => y => x + y
Multiply with a number
const multiply = m => n => n * m
Find volume
const volume = l => w => h => l w h
Find volume of cylinder
const aCylinder = volume­(10­)(1­0)(10)
Currying is a transf­orm­ation of functions that translates a function from callable as f(a, b, c) into callable as f(a)(b­)(c).

Array methods

Double each array element
const map1 = => x *2)
Print each array element­rEach(e => consol­e.l­og(e))
The sum of array elements­duc­e((acc, val) => acc + val)
Filter array
[1, 2, 3].fil­ter(n => n > 2)
You can shorten and simplify your code with the Array methods!

Higher­-Order functions

function greaterThan(n) {
   return x => x > n;
}let greaterThanTwo = greaterThan(2);console.log(greaterThanTwo(5));
In FP, an important concept is Higher­-order functions.

It's a function that accepts (what's typically called) a callback function.
In JS you can benefit by using libraries like Ramda or lodash/f.

Function Compos­ition

var compose = (f, g) => (x) => f(g(x));
Function Compos­ition is an act of compos­ing­/cr­eating functions that allow you to further simplify and compress your functions by taking functions as an argument and return an output.

It may also return another function as its output other than numeri­cal­/string values.


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