Up to now, every variable you have declared can be changed. Set it, then set it again. No problem.

Sometimes you do not want that. You set a value once. You do not want anyone, including future you, changing it later.

Dart has two keywords for that: final and const.

They look similar. They are not the same.

The difference comes down to one question: when does Dart know the value?

Compile time and runtime

Before you can use final and const well, you need two terms.

A compile-time constant is a value Dart can see just by reading your code. Like 3.14159. Or 'hello'. The value is written right there in the source.

A runtime value is one Dart can only figure out while the program runs. Like the result of a function call.

final

A final variable can only be set once. After that, it cannot be reassigned.

String getName() {
  return 'Ali';
}
 
void main() {
  final name = getName();  // set once, accepts a runtime value
  print(name);             // Ali
 
  // name = 'Sara';        // Error: the final variable 'name' can only be set once
}

final accepts runtime values. It does not need to know the value before the program runs. It just says: set it once, and it stays.

const

const is stricter. The value must be a compile-time constant. Dart has to know it before the program ever starts.

void main() {
  const pi = 3.14159;  // Dart knows this before the program runs
  print(pi);           // 3.14159
 
  // pi = 3.14;        // Error: constant variables can't be assigned a value
}

3.14159 is written right there. No function call, no computation. Dart knows it at compile time.

But const will not accept a runtime value:

String getName() {
  return 'Ali';
}
 
void main() {
  final goodName = getName();    // OK: final accepts runtime values
  // const badName = getName();  // Error: must be a constant expression
}

getName() is a function call. Dart can only know its result at runtime. const refuses.

When to use which

Prefer const when the value is a fixed literal: a number, a string, something written directly in the code.

Use final when the value is computed at runtime: a function result, or anything that has to run to produce a value.

If you are not sure, try const first. If Dart rejects it, use final.

Summary