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Lecture 9 - Functional Programming in JavaScript

Today's Agenda

  • Pure Function + Side Effects + Immutability
  • Higher Order Function
  • Function Scope + Closure + Hoisting
  • Callback Function
  • IIFE (Immediate Invoke Function Expression)

Function statement vs Function expression

// function statement
function func() {}

// Function expression
const myFn = function () {};

// Fat Arrow function
const myFatArrowFn = () => {};

Pure Function and Side Effects:

If a function is not able to change any value of a variable, the function is called pure function. If input is same, output is same for forever. For example:

// Pure Function
function sum(a, b) {
	return a + b;
}

sum(10, 20); // 30

Now let's talk about the side effect. If a function can update the value of a variable, it's called side effect. Examples are given below:

// Pure Function
let limit = 100;
function changeLimit(limit) {
	limit = 500;
	return limit;
}

This will not change the value of limit. That is why This is a pure function.

// Side effect
let limit = 100;
function changeLimit() {
	limit = 500;
}

console.log(changeLimit(limit)); // undefined
console.log(limit); // 500

This will change the value of limit. So, it is the example of side effect. There are more examples are given below:

// Pure Function
const arr = [1, 2, 3];
function add(arr, data) {
	arr = [...arr, data];
	return arr;
}
// Side Effect
const arr = [1, 2, 3];
function add(data) {
	arr.push(data);
}
// Impure Function
function log(msg) {
	console.log(msg);
}

This function looks like a pure function, but it is an impure function. Because the console logs are side effects because they're logging out to the console. So if a function consists console logs there is a possibility that the function has some side effects.

Higher Order Function

There are two condition for higher order function.

  • Function can be passed as an argument.
  • Function can be returned from another function.

Example

  • Function can be passed as an argument.

    function randomSum(max) {
	const random1 = Math.floor(Math.random() * max);
	const random2 = Math.floor(Math.random() * max);
	return random1 + random2; // placeholder
}

function randomSub(max) {
	const random1 = Math.floor(Math.random() * max);
	const random2 = Math.floor(Math.random() * max);
	return random1 - random2; // placeholder
}

function randomSqrSum(max) {
	const random1 = Math.floor(Math.random() * max);
	const random2 = Math.floor(Math.random() * max);
	return random1 * random1 + random2 * random2; // placeholder
}

    There are many repetitive codes in the example. To follow the DRY (Don't Repeat Yourself) we can write the functions like this:

    function generateTwoRandNumber(max, cb) {
	const random1 = Math.floor(Math.random() * max);
	const random2 = Math.floor(Math.random() * max);
	const result = cb(random1, random2);
	return result;
}
const cb = function (rand1, rand2) {
	console.log(rand1, rand2);
};
generateTwoRandNumber(100, cb);

console.log(generateTwoRandNumber(1000, (rand1, rand2) => rand1 + rand2));
console.log(generateTwoRandNumber(10, (rand1, rand2) => rand1 * rand2));
console.log(
	generateTwoRandNumber(10, (rand1, rand2) => rand1 * rand1 + rand2 * rand2)
);

    Here generateTwoRandNumber() is a higher order function. Because we pass a function as an argument.

  • Function can be returned from another function.

    function power(p) {
	return function (n) {
		let result = 1;
		for (let i = 1; i <= p; i++) {
			result *= n;
		}
		return result;
	};
}

Hidden Concepts

There are some hidden concepts:

  • Scope

    • Global 
      const a = 10;
function mostOuter() {
	function outer() {
		console.log(a);
	}
}
    • Local 
      function mostOuter() {
	function outer() {
		const a = 10;
		console.log(a);
	}
}
    • Block 
      {
	const notScoped = 'scoped';
	{
		{
			{
				console.log(notScoped);
			}
		}
	}
}

  • Closure: Closure is just a memory, which we can use after a function died.

  • Execution context

    function A(a) {
	console.log('I am A');
}

function B() {
	A();
}

function C() {
	B();
	B();
}
function D() {
	C();
	A();
}

D();

  • Hoisting (Only applicable for var, not applicable for let and const)

    function randomSum(max) {
	const random1 = Math.floor(Math.random() * max);
	const random2 = Math.floor(Math.random() * max);
	t();
	function t() {
		console.log(test);
	}
	var test = 'something';
	t();
	return random1 + random2; // placeholder
}

const r = randomSum(15);

Running a code

There are three steps to run a code

  1. Lexing/Parsing/tokenize
  2. Compile
  3. Run

Defining a Variable

There are two ways to define a variable.

  1. Globally
  2. Locally

IIFE (Immediately Invoke Function Expression)

(function (name) {
	console.log(name);
})('Nayem');

(() => {
	console.log('Test');
})();
  • Use case: We use IIFE to protect our variable from being accessed by anyone. By IIFE we can store our confidential variable.

Important Links

Task

  • Research about Higher Order Function.
  • Create lodash library by immutable way.