PureFunctions.md

title	description
Pure Functions	Pure Functions benifits

Pure functions

Pure vs Impure

const xs = [1,2,3,4,5];

// pure
xs.slice(0,3); // [1,2,3]

xs.slice(0,3); // [1,2,3]

xs.slice(0,3); // [1,2,3]

// impure
xs.splice(0,3); // [1,2,3]

xs.splice(0,3); // [4,5]

xs.splice(0,3); // []

Pure vs Impure

// impure
let minimum = 21;
const checkAge = age => age >= minimum;

// pure
const checkAge = (age) => {
  const minimum = 21;
  return age >= minimum;
};

Side-effects

• changing the file system
• inserting a record into a database
• making an http call
• mutations
• printing to the screen / logging
• obtaining user input
• querying the DOM
• accessing system state

Memoization

const toLowerCase = {
  A: 'a',
  B: 'b',
  C: 'c',
  D: 'd',
  E: 'e',
  F: 'f',
};
toLowerCase['C']; // 'c'

const isPrime = {
  1: false,
  2: true,
  3: true,
  4: false,
  5: true,
  6: false,
};
isPrime[3]; // true

Memoization

const squareNumber = memoize(x => x * x);

squareNumber(4); // 16

squareNumber(4); // 16, returns cache for input 4

squareNumber(5); // 25

squareNumber(5); // 25, returns cache for input 5

Memoization

const memoize = (f) => {
  const cache = {};

  return (...args) => {
    const argStr = JSON.stringify(args);
    cache[argStr] = cache[argStr] || f(...args);
    return cache[argStr];
  };
};

Portable / Self-documenting

// impure
const signUp = (attrs) => {
  const user = saveUser(attrs);
  welcomeUser(user);
};

// pure
const signUp = (Db, Email, attrs) => () => {
  const user = saveUser(Db, attrs);
  welcomeUser(Email, user);
};

Functional Programming book

Mostly-adequate