# Dart Extensions to Add Methods to Existing Classes

> Use the extension keyword in Dart to add custom methods, getters, setters, and operators to any existing class or data type, including DateTime

**URL:** https://sentry.io/answers/extensions-in-dart/

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## The Problem

It's not much of a solution to a problem, but a nice convenience.
Extensions in Dart allow you to develop custom functionalities on
any library or data structure, including your own. So how do we go
about them?

## The Solution

There's a specific keyword in Dart used to create extensions:
`extension`. When creating an extension we also need to specify on
which data type it should apply. Let's say we want to make a few
useful `DateTime` extension methods. We should create a separate
file and create our date extensions inside:

```dart
// date_extensions.dart

extension DateExtensions on DateTime {
	// ...
}
```

Let's create an extension method that checks whether one date
is at the same day as another:

```dart
extension DateExtensions on DateTime {
	bool isSameDay(DateTime other) {
    return this.year == other.year &&
        this.month == other.month &&
        this.day == other.day;
  }
}
```

Note: I've put `this` so you can see how the comparison happens.
The IDE will most likely warn you: `Don't access members with this unless avoiding shadowing.`

We can use our new extension method like so (don't forget
to import your dart extension file):

```dart
date1.isSameDate(date2)
```

The `this` in our extension method refers to the `date1` variable.

Aside from methods, we can also create getters, setters and operators
as well. Here's a getter method that returns a date in a specific
format:

```dart
extension DateExtensions on DateTime {
	// ...
	get formattedDate {
		DateFormat format;
		if (year != DateTime.now().year) {
		  format = DateFormat("E, d MMM yyyy");
		// we can also use other extension methods inside
		} else if (isToday()) {
		  return "Today";
		// we can also use other extension methods inside
		} else if (isYesterday()) {
		  return "Yesterday";
		} else {
		  format = DateFormat("E, d MMM");
		}

		return format.format(this);
	}
}
```

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*Source: [sentry.io/answers/extensions-in-dart/](https://sentry.io/answers/extensions-in-dart/)*