What's the difference between `var`, `final`, and `const` in Dart?

var declares a variable that can be reassigned. final declares a variable that is assigned exactly once at runtime. const declares a compile-time constant: its value has to be known before the program runs. The recommendation is to use final by default and const when the value is fixed at compile time.

• var: tells Dart "infer the type for me." The value can be reassigned as many times as you want.
• final: the variable is assigned once and can't be reassigned. The value can be computed at runtime (for example, the current time or something coming from a function).
• const: a compile-time constant. The value must be known before the program runs, so it can't depend on anything computed at runtime.

The key difference between final and const: final is resolved when the program runs; const is resolved when it compiles. A const object is also "frozen" (it's deeply immutable), and Dart reuses it in memory (canonicalization).

Rule of thumb: use final by default. If the value is literally fixed and known at compile time, bump it up to const.

void main() {
  var counter = 0;
  counter = 1; // OK: var can be reassigned

  final name = "Mike";
  // name = "Other"; // Error: you can't reassign a final

  final now = DateTime.now(); // OK: computed at runtime
  print(now);

  const pi = 3.1416; // value known at compile time
  // const invalid = DateTime.now(); // Error: DateTime.now() is runtime

  // const freezes the contents (truly immutable)
  const list = [1, 2, 3];
  // list.add(4); // Runtime error: the list is immutable
  print(list);
}

Believing that final freezes the contents of an object. It doesn't: final only prevents reassigning the variable. A List declared with final can still mutate its contents (.add(), .remove()). For the contents to be immutable you need const.

final numbers = [1, 2, 3];
numbers.add(4); // OK: final allows mutating the contents
print(numbers); // [1, 2, 3, 4]

"var allows reassignment. final is assigned only once, but its value can be computed at runtime. const is a compile-time constant: the value has to be known before the program runs, and it also freezes the object so it's immutable. I use final by default and const when the value is fixed at compile time, like in Flutter's const constructors, which help performance."