Functional Programming

Author: Doe Hoon LEE

한글버전 <- 클릭!

English Ver.

What is it about?

1. Pure Functions

• Functions should not have side effects

• Do not use globally defined variables to compute its output

• Functions should compute output depending on its input (=> with the same input, it always returns the same output)

NOT FUNCTIONAL

const name = "Green Apple"
const num = 19;
console.log(`${name} is ${name} years old`); // => "Green Apple is 19 years old"

Functional

function background(name, age) {
    console.log(`${name} is ${name} years old`);
}

background("Green Apple",19); // => "Green Apple is 19 years old"

2. Higher Order Functions

• Higher-Order function takes a function as an argument or returns a function or both

3. Avoiding Side-Effects

• Do NOT modify global variables

• Do NOT write/log to screen/console

• Do NOT write to network

• Do NOT triger any external process

4. Avoiding Mutability

• Data should not be modified after it is created

5. Avoiding Iteration (like for loop)

• Use higher order functions like map(), reduce(), filter(), forEach() instead of for loop

methods	Description
map()	Creates a new array with elements resulted from calling a provided function
reduce()	Takes an array and reduces to a single value
filter()	Creates a new array with elements that passes the given condition
forEach()	Calls a function for each element visited

What is so good about functional programming?

• Enhanced readability

• Easier to test

• More predictable

• Better reusability