Core Principles

Pure Functions

A function is pure if:

• It always produces the same output for the same input.
• It has no side effects (does not modify external state).

Pure: No external dependency, no mutation.

const add = (a: number, b: number) => a + b;
console.log(add(2, 3)); // 5

Impure example (modifies external state):

let total = 0;
const addToTotal = (num: number) => (total += num);

Immutability

• Avoid modifying existing objects/arrays; instead, return a new copy.

const arr = [1, 2, 3];
const newArr = [...arr, 4]; // [1, 2, 3, 4]

const person = { name: "Alice", age: 25 };
const updatedPerson = { ...person, age: 26 }; // New object, age updated

Function Composition

• Combining simple functions to build more complex logic.

const toUpper = (str: string) => str.toUpperCase();
const exclaim = (str: string) => str + "!";
const shout = (str: string) => exclaim(toUpper(str));

console.log(shout("hello")); // "HELLO!"

Key Techniques

Closures

A function that "remembers" variables from its outer scope.

const counter = () => {
  let count = 0;
  return () => ++count;
};

const increment = counter();
console.log(increment()); // 1
console.log(increment()); // 2

Currying