Extensions in Dart

Lazar Nikolov

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&aspo;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:

// date_extensions.dart

extension DateExtensions on DateTime {
    // ...
}
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Let’s create an extension method that checks whether one date is at the same day as another:

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

date1.isSameDate(date2)
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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:

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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