Dart Extensions Part 1: Introduction, Extending Custom Classes, Chaining Extensions

Dart Extensions Part 1: Introduction, Extending Custom Classes, Chaining Extensions

Extensions are one of the coolest features of dart. While I heard a lot about them, I never was able to grasp them properly. So in here we are going to look at 3 Tutorials that'll help us with Dart extensions

As always if you want to follow along you can find the source code here: dart-concepts-code

Tutorial 1: Introduction to Dart Extensions

What are Dart Extensions?

Dart extensions allows us to add new functionality to a class without modifying them. This feature enables us to extend the capabilities of built-in and custom classes.

Basic Syntax: Here's the basic syntax for creating an extension in Dart:

extension ExtensionName on Type { 
// Add methods or properties here 


Let's create a simple extension on the String class to add a method capitalize that capitalizes the first letter of the string.

extension StringExtensions on String {
  String capitalize() {
    if (this.isEmpty) {
      return this;
    return this[0].toUpperCase() + this.substring(1);

void main() {
  String name = "harish";

So in here we got two things:

  • The StringExtensions extension adds a new method capitalize to the String class.

  • The capitalize method checks if the string is empty. Otherwise it capitalizes the first letter and concatenates it with the rest of the string.

You can find the above code in here: Tutorial 1: String Extension

Assignment 1:

Create an extension on the String class that adds a method isPalindrome to check if the string is a palindrome.

You can find the solution in here: Assignment-1 Solution

Tutorial 2: Extending Custom Classes

In here we are going to cover the following topics:

  • Extending custom classes

  • Using extensions to add utility methods

  • Scope and limitations of extensions

Creating a Custom Class:

First let's add a simple Person with properties name and age

class Person {
  String name;
  int age;
  Person(this.name, this.age);

Extending the Custom Class

Now, let's write an extension to add a method greet that returns a greeting message including the person's name.

extension PersonsExtensions on Person {
  String greet() {
    return "Hello my name is $name and I am $age years old.";

void main() {
  Person person = Person('Harish', 28);


So in the above code:

  • The PersonExtensions extension adds a new method greet to the Person class.

  • The greet method constructs a greeting message using the name and age properties of the Person instance.

Scope and Limitations of Extensions

  • Scope: Extensions can only add methods and getters. They cannot add instance variables or modify the internal state of the class directly.

  • Limitations: Extensions do not have access to private members of the class they are extending. They can only work with public members.

You can find the code here: Tutorial 2 Custom Class Extension

Assignment 2:

Create a custom class Book with properties title and author. Write an extension to add a method description that returns a string describing the book with its title and author.

You can find the solution here: Assignment 2 Solution

Tutorial 3: Chaining Extensions

In here we are going to cover:

  • Method chaining with extensions

  • Creating fluent interfaces using extensions

  • Examples of chaining methods

Example: Chaining Methods

Let's create an extension on the List class.

We'll add two methods:

  • doubleEach (doubles each element in a list of integers)

  • sum (returns the sum of all elements).

We will use method chaining to double the elements and then get the sum.

extension ListExtensions on List<int> {
  List<int> doubleEach() {
    for (int j in this) {
      j = j * 2;

    return this;

  int sum() {
    int total = 0;

    for (int value in this) {
      total += value;

    return total;

void main() {
  List<int> numbers = [1, 2, 3, 4, 5];

  int result = numbers.doubleEach().sum();


Perfect in here:
The ListExtensions extension adds two methods to the List<int> class:

  • doubleEach: Doubles each element in the list and returns the modified list.

  • sum: Returns the sum of all elements in the list.

You can find the source code here: Tutorial 3: List Extensions

Assignment 3:

Create an extension on the String class to add methods reverse (reverses the string) and capitalizeEachWord (capitalizes the first letter of each word). Use method chaining to reverse the string and then capitalize each word.

Solution: Assignment 3 Solution

Next Tutorial:

In the next tutorial we'll look at

Tutorial 4: Extensions for Collections

  • Working with collections (List, Map, Set)

  • Adding utility methods for collections

  • Examples of useful collection extensions

Tutorial 5: Advanced Extensions and Best Practices

  • Advanced use cases of extensions

  • Combining extensions with other Dart features

  • Best practices and performance considerations

So Stay tuned and do subscribe to my blog for more updates.

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