When developing with Dart, especially when using third-party packages or following tutorials, you’ve probably come across the .. symbol followed by property or method names. But what exactly does this .. or Cascade Notation do?

Cascade Notation

Cascade Notation is a form of syntactic sugar—a syntax feature that makes coding more enjoyable and concise. That’s why it’s called a notation, not an operator: it’s part of Dart’s syntax itself. In many cases, it also makes code more readable.

Before diving deeper into how this notation works, let’s look at some examples to build our understanding.

Original

class Human {
  double weight;
  double height;
  Human({this.weight, this.height});
  void sayHello() {
    print("Hello");
  }
}

void main() {
  final mike = Human(weight: 78.0, height: 183.0);
  mike.height = 176.5;
  mike.weight = 82.3;
  mike.sayHello();
}

Modified using Cascade Notation

class Human {
  double weight;
  double height;
  Human({this.weight, this.height});
  void sayHello() {
    print("Hello");
  }
}

void main() {
  final mike = Human(weight: 78.0, height: 183.0);
  mike..height = 176.5
    ..weight = 82.3
    ..sayHello(); // print -> Hello
}

As you can see from the examples, we can eliminate repetitive references to mike by using Cascade Notation. The core mechanism behind Cascade Notation is that it always returns a reference to the same instance after performing some operation. For example, in the code above, mike..height = 176.5 is equivalent to calling mike.height = 176.5 and then returning mike for the next line. This means ..weight = 82.3 effectively becomes mike.weight = 82.3.

The primary use case for this feature is initializing properties right after creating a fresh object.

One important caveat when using Cascade Notation: when used with functions that return a value (non-void return types), that returned value will be ignored. So if you need that return value for something, it will be discarded.

Learn more about Cascade Notation in the Official Guide

Thanks for reading

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