When writing test expectations for functions that throw, we need a bit careful.
For example, consider this simple test:
test('function throws', () {
expect(
someFunctionThatThrows(),
throwsStateError,
);
});In this case, the test will fail, because someFunctionThatThrows() is evaluated before the expect function is called.
This is equivalent to writing the test like this:
test('function throws', () {
// if the function throws, the test will fail immediately
final result = someFunctionThatThrows();
// and this line will never be exectuted
expect(
result,
throwsStateError,
);
});To fix this, we can pass an anonymous function as an argument to the expect method:
test('function throws', () {
expect(
// function argument, will be evaluated *inside* the expect function
() => someFunctionThatThrows(),
throwsStateError,
);
});Or even better, we can use a tear-off and pass someFunctionThatThrows directly as an argument (without calling it):
test('function throws', () {
expect(
someFunctionThatThrows,
throwsStateError,
);
});This is safer, and more concise too! 👌
For more info about tear-offs, see this rule about effective Dart usage.
Happy coding!
