Creating Custom JavaScript Functions: Best Practices and Examples

JavaScript functions are an essential part of any web development project. They allow developers to encapsulate blocks of code and reuse them throughout their codebase. Creating custom JavaScript functions can make your code more efficient, readable, and maintainable.

In this blog post, we'll cover some of the best practices for creating custom JavaScript functions and provide some examples to help you get started.

Best Practices for Creating Custom JavaScript Functions

1. Write functions that are specific and reusable

The purpose of creating custom JavaScript functions is to simplify and reuse code throughout your project. It's important to ensure that each function is specific to a particular task and can be used in multiple places in your codebase.

For example, if you have a function that formats a date, make sure it can be reused throughout your project. You may need to format dates in several different places, so it's important to have a function that can handle this task.

2. Use function parameters to make your functions more flexible

Function parameters allow you to make your functions more flexible and reusable. They allow you to pass in different values to the function, depending on the context in which it is used.

For example, if you have a function that calculates the area of a rectangle, you can pass in the width and height as parameters. This allows you to use the same function for rectangles of different sizes.

3. Use return statements to make your functions more versatile

Return statements are used to send data back to the calling code. They allow your functions to be more versatile and return different types of data depending on the input values.

For example, a function that calculates the average of an array of numbers could return the average as a number or return an error message if the input is not an array.

4. Use comments to explain your functions

Comments are a great way to explain how your functions work and what they are used for. They can help other developers understand your code and make it easier to maintain in the future.

Make sure to include comments that explain what your function does, what parameters it takes, and what it returns.

Examples of Custom JavaScript Functions

1. A function to format a date

function formatDate(date) {
  const options = { year: 'numeric', month: 'long', day: 'numeric' };
  return new Intl.DateTimeFormat('en-US', options).format(new Date(date));

console.log(formatDate('2022-02-24')); // February 24, 2022

This function takes a date string as input and returns a formatted date string in the format "Month Day, Year".

2. A function to calculate the area of a rectangle

function calculateArea(width, height) {
  return width * height;

console.log(calculateArea(5, 10)); // 50

This function takes the width and height of a rectangle as input and returns the area of the rectangle.

3. A function to calculate the average of an array of numbers

function calculateAverage(numbers) {
  if (!Array.isArray(numbers)) {
    return 'Error: Input is not an array';
  const sum = numbers.reduce((acc, val) => acc + val, 0);
  return sum / numbers.length;

console.log(calculateAverage([1, 2, 3, 4, 5])); // 3
console.log(calculateAverage('not an array')); // Error: Input is not an array

This function takes an array of numbers as input and returns the average of those numbers. If the input is not an array, it returns an error message.

4. A function to check if a string contains a specific word

function containsWord(string, word) {
  return string.includes(word);

console.log(containsWord('The quick brown fox', 'fox')); // true
console.log(containsWord('The quick brown dog', 'fox')); // false

This function takes a string and a word as input and returns true if the string contains the word, and false otherwise.

5. A function to generate a random number between two values

function getRandomNumber(min, max) {
  return Math.floor(Math.random() * (max - min + 1) + min);

console.log(getRandomNumber(1, 10)); // a random number between 1 and 10

This function takes a minimum and maximum value as input and returns a random integer between those two values.

6. A function to capitalize the first letter of each word in a string

function capitalizeFirstLetter(string) {
  return string.split(' ').map(word => word.charAt(0).toUpperCase() + word.slice(1)).join(' ');

console.log(capitalizeFirstLetter('the quick brown fox')); // The Quick Brown Fox

This function takes a string as input and returns the same string with the first letter of each word capitalized.


Creating custom JavaScript functions is an essential skill for any web developer. By following best practices and using examples like the ones we've shown here, you can create functions that are specific, reusable, flexible, and versatile. With these tools, you'll be able to write code that is more efficient, readable, and maintainable.

Did you find this article valuable?

Support Mikaeel Khalid by becoming a sponsor. Any amount is appreciated!