Basics

Comments and Scope

A // comment is reader-only text that Dart skips at runtime. Each { ... } block introduces a new scope: a var declared inside disappears at the closing brace, while names from the enclosing scope stay visible.

Program

Play the program to enter an if block, build a label from inner state, then watch that inner name go out of scope.

comments_scope.dart
Replay: real traced execution (multi-file project)
void main() {
  var label = 'outer';
  // a reader-only comment; this does not run
  if (label == 'outer') {
    var detail = 'inner';
    label = '$label/$detail';
  }
  print(label);
}

  1. label ← outer

    1void main() {2  var label = 'outer';3  // a reader-only comment; this does not run
    values this stepouterlabel

  2. condition ← true

    3// a reader-only comment; this does not run4if (label == 'outer') {5  var detail = 'inner';
    values this steptrueconditionouterlabel

  3. detail ← inner

    4if (label == 'outer') {5  var detail = 'inner';6  label = '$label/$detail';
    values this stepinnerdetail

  4. label ← outer/inner

    5  var detail = 'inner';6  label = '$label/$detail';7}
    values this stepouter → outer/innerlabelinnerdetail

  5. print(label);

    7  }8  print(label);9}
    outputouter/inner
    values this stepouter/innerlabel

Follow the Scope

  1. label starts as outer.
  2. The condition label == 'outer' is true.
  3. Inside the block, detail is inner.
  4. label becomes outer/inner.
  5. The program prints outer/inner. | moment | visible value | | --- | --- | | start | label = outer | | condition | true | | inside block | detail = inner | | after update | label = outer/inner | | stdout | outer/inner |
// comment Lines starting with `//` are for readers only; Dart skips them at runtime.
block scope `var detail` declared inside the `{ ... }` block exists only there; the outer `label` stays visible.
after the brace Past the closing `}` `detail` is gone, but `label` keeps its updated value.

Exercise: comments_scope.dart

Reproduce outer/inner, then identify which value exists only inside the block.