Different types of Functions in JavaScript

Photo by Markus Spiske on Unsplash

A JavaScript function is a block of code designed to perform a particular task.

MDN Says:

Functions are one of the fundamental building blocks in JavaScript. A function is a JavaScript procedure — a set of statements that performs a task or calculates a value. To use a function, you must define it somewhere in the scope from which you wish to call it.

A JavaScript function is executed when “something” invokes it (calls it).

Example:

function square(x) {
  return x * x;
}
square(10); // 100

Basic Syntax:

function validFunctionName(parameter) {
  return statement;
}

A function can have multiple parameters or no parameters at all. In the following example, bark does not list any parameter names, whereas power lists two:

bark( )

function bark() {
  return "woof-woof";
}
bark(); //   woof-woof

power( )

function power(base, exponent) {
  let result = 1;
  for(let count = 0; count < exponent; count++) {
    result *= base;
  }
  return result;
}
power(2, 10); // 1024

Function Expression:

A Function Expressions defines a named or anonymous function. An anonymous function is a function that has no name.

var fullName = function(firstName, lastName) {
 return `${firstName} ${lastName}`;
}
fullName("Jamal", "Uddin"); // Jamal Uddin

Arrow Function:

An Arrow Function Expression is a shorter syntax for writing function expressions. Arrow functions do not create their own value.

We can write the arrow function in multiple ways:

First:

it just looks like a regular function expression but have arrow (=>) key.

const double = (value) => {
  return value * 2
}
double(10); // 20

Second:

Omit the return keyword

const double2 = value => value * 2;
double2(10); // 20

Third:

If our function have no parameter

const noise = () => console.log("Pling");
noise(); // Pling

or

const noise2 = _ => console.log("Pling");
noise2(); // Pling

Fourth:

If we have two or more parameter, you can must be used parenthesis

const addAll = (x, y, z) => x + y + z;

addAll(10, 20, 30); // 60

Fifth:

We can use default value in our parameters

const multiply = (a = 2, b = 3, c = 1) => a * b * c;
multiply(2, 2, 2); // 8
multiply(2, 2);    // 4
multiply(3);       // 9
multiply();        // 6

JavaScript is extremely broad-minded about the number of arguments you pass to a function. If you pass too many, the extra ones are ignored. If you pass too few, the missing parameters get assigned the value undefined.

Return:

Remember, the return keyword can ONLY be used inside of a function. let's take a look at another example.

function returnOnlyOnce(){
  return "Hello";
  return "Goodbye";
}
returnOnlyOnce(); // "Hello"

We see from this example that the return keyword can only be executed once in a function. Once it is executed, the function is complete and no other lines of code will be executed.

Function Shorthand methods:

Shorthand method definition can be used in a method declaration on object literals and ES6 classes. We can define them using a function name, followed by a list of parameters in a pair of parenthesis (para1, ..., paramN) and a pair of curly braces { ... } that delimits the body statements.

The following example uses shorthand method definition in an object literal:

const fruits = {  
  items: [],
  add(...items) {
    this.items.push(...items);
  },
  get(index) {
    return this.items[index];
  }
};
fruits.add('mango', 'banana', 'guava');  
fruits.get(1); // banana

add() and get() methods in fruits object are defined using short method definition. These methods are called as usual: fruits.add(...) and fruits.get(...).

Generator Function:

ES6 introduced a new way of working with functions and iterators in the form of Generators (or generator functions). A generator is a function that can stop midway and then continue from where it stopped. In short, a generator appears to be a function but it behaves like an iterator.

Note: async/await is based on generators. Read more here.

Example:

function * generatorFunction() { 
  yield 'Hello, ';
console.log('I will be printed after the pause');  
  yield 'World!';
}
const generatorObject = generatorFunction();
console.log(generatorObject.next().value);
console.log(generatorObject.next().value);
console.log(generatorObject.next().value);
// output should be following below.
// Hello, 
// I will be printed after the pause
// World!
// undefined

Function with: new Function

The Function constructor creates a new Function object.

var sum = new Function('a', 'b', 'return a + b');
console.log(sum(2, 6)); // 8

Source of truth:

• MDN JavaScript Function
• Eloquent JavaScript functions chapter
• w3schools

P.S: English is not my native language and this is my very first English article so if you have found any mistake please pardon me also hearts to encourage me to write more articles.

Happy Programming! :)

Who Am I?

This is Md. Jamal Uddin working as a Software Developer based in Dhaka, Bangladesh. I love to learn new things and share them with others. Playing with cutting technologies is my hobby and working with legacy is my day job :). Connect me on Twitter and LinkedIn

Comments

[Guillaume Martigny]

Nice article. Small comment, on the third arrow function example, when you write :

const noise2 = _ => console.log("Pling");

There's an unused parameter named _ instead of no parameter. Both are valid anyway, just wanted to point out the difference ;)

[Md. Jamal Uddin]

Yeah! It's intentional. Only for showing variation. Thanks so much for your comment.

[David Wickes]

Very comprehensive!

Couple of typos:

Your default parameters examples has ; instead of , between the arguments

const multiply = (a = 2, b = 3, c = 1) => a * b * c;

You're missing a ' on your new Function example:

var sum = new Function('a', 'b', 'return a + b');

(I love that you introduce the Function constructor by the way - hours of fun for everyone there)

[Md. Jamal Uddin]

Bull's eye man! truly I have read my post more than 10 times :) and two of my friends proofread it but no one figured it out. I will be more prepared next time. thanks.

[victordeflos]

i am a beginner in coding i am currently finishing frond end courses and for the final test i need an idea to be able to make some program in react. please help or some page on the web where I could find examples and printed codes. thank you very much

[Waqar Mohammad]

Nice article dude. Your English is great too. Keep it up

[Md. Jamal Uddin]

Thanks

[Thomas Bishop]

Very helpful article and admirably free of waffle. Nice work :)

[Md. Jamal Uddin]

Thanks

[Tamal Chowdhury]

Very in depth and helpful.

[Md. Jamal Uddin]

Thanks man!😍

[Álvaro Cáceres Muñoz]

Nice article! You managed to make me understand generators for the first time XD.

By the way, could you please give me a practical use case of the Function consctructor? Thank you

[Md. Jamal Uddin]

here is an example Function constructor with implementing a class and you know JavaScript class is a syntactic sugar over the function structure.

class Animal {
  constructor(name, energy) {
    this.name = name
    this.energy = energy
  }
  eat(amount) {
    console.log(`${this.name} is eating.`)
    this.energy += amount
  }
  sleep() {
    console.log(`${this.name} is sleeping.`)
    this.energy += length
  }
  play() {
    console.log(`${this.name} is playing.`)
    this.energy -= length
  }
}

const leo = new Animal('Leo', 7)

wanna know more, please check out this stackoverflow thread

Thanks!

[Md Abu Taher]

Go forth!

[Md. Jamal Uddin]

Thanks bro 😍 to see you here.

[victordeflos]

and whether there is a torrent site for developers or a community.