Scope

Scope is the region of code where a name can be used. Every pair of curly braces creates a new scope.

Real-world analogy: scopes are nested rooms. From an inner room you can see into the rooms around it, but people outside can't see in.

Example

const taxRate = 0.2;          // top level: visible everywhere in the file

double total(double price) {
  final tax = price * taxRate;  // inner code can see outer names
  return price + tax;
}

void main() {
  print(total(100));

  if (true) {
    final message = 'inside';
    print(message);
  }
}

120.0
inside

• taxRate is declared at the top level, so every function can use it.
• tax exists only inside total.
• message exists only inside the if braces.

Using a name outside its scope is a compile error:

void main() {
  if (true) {
    final message = 'inside';
  }
  print(message);
}

hello.dart:5:9: Error: Undefined name 'message'.

Library privacy

Top-level names starting with an underscore, like _cache, are private to their library (usually one file). Other files can't see them — Dart's simple form of access control.

Keep scope small

Declare values close to where they're used, and pass values into functions as parameters instead of reading many top-level variables. Mutable top-level variables in particular make programs hard to follow.