Understanding JavaScript Functions: Types and Usage

A Comprehensive Guide to JavaScript Function Types and Their Applications

Harendra Kumar
6 min readSep 27, 2023

In JavaScript, a function is a block of code that may be called repeatedly to do a specified purpose. The function keyword is used to build functions, which is followed by the function’s name and a list of parameters in brackets. Curly braces surround the function’s code body. Simply supply the function’s name followed by brackets to invoke it. When you call the function, you must give the right amount and type of arguments.

Type of Functions In Javascript

  1. Regular Functions
  2. Arrow Functions
  3. Constructor Functions
  4. Method Functions
  5. Callback Functions
  6. Async Functions

Regular Functions in Javascript

  • Regular functions are defined using the function keyword followed by the function name and parameters in parentheses.
  • For example:
function add(a, b) {
return a + b;
}
  • The code inside the function body (between the curly braces {}) will run when the function is called.
  • Functions can accept parameters (also known as arguments) which are passed into the function when it is invoked.
  • Parameters act as local variables that are only accessible within the function body.
  • Functions can optionally return a value using the return keyword. Anything after a return statement will not execute.
  • Functions in JavaScript are first-class objects — they can be assigned to variables, passed into other functions, and returned from functions like any other object.
  • Regular functions follow the typical function execution context — the this keyword refers to the global object.
  • They are useful for organizing and reusing code, abstracting logic into reusable chunks, and dividing programs into smaller testable units.
  • Common uses include math operations, data validation, DOM manipulation, and abstracting repetitive tasks into reusable blocks of code.
  • Regular functions are the standard way to define functions in JavaScript before arrow functions were introduced.

Arrow Functions in Javascript

  • Arrow functions were introduced in ES6 as a shorter syntax for writing regular functions, especially for anonymous ones.
  • The basic syntax is:
const funcName = (params) => expression;
  • The “fat arrow” => is used instead of the function keyword. No function keyword is required.
  • If there is only one parameter, the parentheses can be omitted:
const double = num => num * 2;
  • For multiline arrow functions, curly braces are required and the return keyword can be omitted:
const greet = (name) => {
console.log(`Hello ${name}!`);
}
  • Arrow functions lexically bind the this keyword, meaning this refers to the enclosing context.
  • They don’t have their own this, arguments, super, or new.target.
  • Useful for short callback functions and avoiding issues with this binding in regular functions.
  • Generally more concise than regular functions, especially for one-line expressions.
  • Not suitable for all cases, like constructor functions or functions that require their own this context.
  • Arrow functions are anonymous but can be named for recursion or debugging purposes.

So in summary, they provide a shorter syntax alternative to regular functions in many cases.

Constructor Functions in JavaScript

  • Constructor functions are used to create user-defined objects through the new keyword.
  • They are defined like regular functions but are intended to be called with new.
  • The function name starts with a capital letter by convention to distinguish it as a constructor.
  • Inside the constructor, the this keyword refers to the newly created object.
  • Properties and methods can be assigned to this to become part of the object.
  • Constructors typically define properties that are common to all objects.
  • To create an object, the constructor is called with new, like:
function Person(name) {
this.name = name;
}
const person = new Person('John');
  • new creates a blank object, sets its constructor property, binds this to it, and returns it if no explicit return is given.
  • Constructors enable object-oriented patterns like inheritance and polymorphism.
  • They establish a prototype for the objects they create to share methods.
  • Constructor functions are essential for creating user-defined objects in JavaScript.

So in summary, constructor functions are used to initialize new objects through the new keyword and this context.

Method Functions in JavaScript:

  • Method functions are functions that are defined within objects as properties.
  • They allow objects to encapsulate behavior alongside properties and data.
  • Methods are defined using the same function syntax but assigned to a property instead of standalone.
  • For example:
const person = {
name: 'John',
greet() {
return `Hello, my name is ${this.name}!`;
}
};
  • Here greet() is a method defined on the person object.
  • Methods receive the object they are called on via their this keyword.
  • This allows methods to access other properties and methods of the same object.
  • Methods are commonly used to define behaviors that objects can perform.
  • They are called using object.method() syntax like person.greet().
  • JavaScript supports defining methods with various syntaxes like object literals, prototypes, and constructor functions.
  • Methods are essential for encapsulating actions within objects and modeling real-world entities and their behaviors.

So in summary, method functions are functions assigned as properties within objects to define the behaviors and actions those objects can perform.

Callback Functions in JavaScript:

  • A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.
  • Callbacks allow asynchronous code to run after an operation completes, without blocking further execution.
  • For example, passing a callback to a function that fetches data from a server:
function getData(callback) {
// async operation
callback(data);
}

getData(function(data) {
// do something with data
});
  • The callback function is invoked by the outer function after the async operation finishes.
  • This makes code asynchronous and non-blocking.
  • Common uses of callbacks include event handlers, asynchronous operations like AJAX requests, and iterating arrays.
  • Callbacks must always be anonymous functions or the function reference will be invoked immediately.
  • Pyramid or “callback hell” can occur with deeply nested callbacks, which is avoided with promises.
  • Understanding how callbacks work is fundamental to asynchronous JavaScript programming.

So in summary, callback functions allow asynchronous code to be executed after an operation completes by passing functions as arguments to other functions.

Async Functions in JavaScript

  • Async functions were introduced in ES2017 to write asynchronous code in a cleaner way compared to callbacks or promises.
  • Async functions are defined using the async keyword before the function keyword:
async function myFunction() {
// async code
}
  • Inside async functions, await can be used to pause execution until a Promise settles.
  • Await makes async functions behave synchronously, resuming after the Promise is fulfilled.
  • This avoids callback hell and allows async code to be written line by line.
  • Async functions always return a Promise, even if no await is used.
  • They can be used with try/catch blocks to handle errors.
  • Common uses include I/O operations, data fetching, and any code that needs to pause execution.
  • Async/await improves readability of async code compared to callbacks/promises alone.
  • Async functions don’t block other code from running like regular functions.
  • Understanding async/await is important for modern asynchronous JavaScript programming.

So in summary, async functions provide a cleaner way to write asynchronous code using async/await rather than callbacks or nested promises.

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Harendra Kumar

Hello, I am full stack software developer from last 6+ years, I love to learn and share about programming.