dart: Mutability of lists

By default, dart lists are mutable and it might lead to code that is hard to debug and predict.

void main() {
  var a = [0,1, 2,3] ;
  var b = a;
  b.add(4);
  print(a);
}
// prints [0, 1, 2, 3, 4]

A change (calling add method) on b causes a ripple effect on the variable a.

There are different ways to prevent this. One way is to use the spread operator.

void main() {
  var a = [0,1, 2,3] ;
  var b = [...a];
  b.add(4);
  print(a);
}
// prints [0, 1, 2, 3]

To prevent any mutation on variable a, we can make it unmodifiable.

import 'dart:collection';
void main() {
  var a = UnmodifiableListView([0,1, 2,3]);
  var b = [...a];
  b.add(4);
  // a.add(5); Unsupported operation: Cannot add to an unmodifiable list
  print(a);
}

If we try to change the list a, we will get error.