What assert do in dart?


Asked by venky r on June 11, 2019 (source).

I just want to know what's the use of assert in a Dart. I was tried to figure it out by myself but I'm not able to do that. It would be great if someone explains to me the use of assert.


Question answered by Esen M (source).

Main purpose of assert is testing conditions during debugging/development.

Let's think about a real example:

class Product {
    required this.id,
    required this.name,
    required this.price,
  })  : assert(id > 0),
        assert(price > 0.0);

  final int id;
  final String name;
  final double price;
  final String? size;
  final String? image;
  final int? weight;

We have a Product class and fields like id, name and price are mandatory but other fields can be handled by generic values as you guess. By asserting required fields, you'll test this data class during debugging/development. Keep in the mind, all asserts ignored in release/production mode;

From the dart.dev#assert:

In production code, assertions are ignored, and the arguments to assert aren’t evaluated.

Comparing to writing tests, even though they are not the same thing, asserts can be very handy with minimal effort, so be generous for writing asserts especially if you don't write tests, it usually rewards you.

Additionally, since constants like kDebugMode, kReleaseMode are part of package:flutter/foundation.dart, another common use case for assert is debugMode specific codes in Non-Flutter applications. Let's have a look this code:

bool get isDebugMode {
  bool value = false;
  assert(() {
    value = true;
    return true;
  return value;

At first it may look confusing but it's a tricky but simple code. An anonymous callback always returns true, so we don't throw any error in any case. Since compiler eliminates the assert statements in release mode, that callback only run in debug mode and mutate the value variable.

Nullable Example

For the versions of Dart before 2.12, your typical example should be look like this:

import 'package:meta/meta.dart';

class Product {
  final int id;
  final String name;
  final int price;
  final String size;
  final String image;
  final int weight;

  const Product({
    @required this.id,
    @required this.name,
    @required this.price,
  }) : assert(id != null && name != null && price != null);