What's the recommended way in Dart: asserts or throw Errors

Question

Dart explicitly makes a distinction between Error, that signals a problem in your code's logic and should never happen and should never be caught and Exceptions that signal a problem based on run-time data.

I really like this distinction but I wonder when should I then use assert() functions?

Answer 1

Asserts are ways to perform code useful in development only, without hindering the performances of release mode – usually to prevent bad states caused by a missing feature in the type system.

For example, only asserts can be used to do defensive programming and offer a const constructor.

We can do:

class Foo {
  const Foo(): assert(false);
}

but can't do:

class Foo {
  const Foo() { throw 42; }
}

Similarly, some sanity checks are relatively expensive.

In the context of Flutter, for example, you may want to traverse the widget tree to check something on the ancestors of a widget. But that's costly, for something only useful to a developer.

Doing that check inside an assert allows both performance in release, and utility in development.

assert(someVeryExpensiveCheck());