Setting up the checked and production modes

Open your app in Dart Editor and select the startup web page or Dart script, usually web\index.html.

Scripts that are started on the command line (or in a batch file) with the dart command run in the Dart VM and thus in the production mode. If you want to run the Dart VM in the checked mode, you have to explicitly state that with the following command:

dart –c script.dart or: dart --checked script.dart 

You can start Dartium (this is Chromium with the Dart VM) directly by launching the Chrome executable from dart\chromium; by default, it runs Dart Editor in the production mode. If you would like to start Dartium in the checked mode, you can do this as follows:

Verify this setting by going to the following address in the Chrome browser that you just started chromium://version.

When a web app runs in the Dart VM in Chrome, it will run in the production mode, by default.

In the checked mode, types are checked by calling assertions of the form assert (var1 is T) to make sure that var1 is of type T. This happens whenever you perform assignments, pass parameters to a function, or return results from a function.

However, Dart is a dynamic language where types are optional. That's why the VM must, in the production mode, execute your code as if the type annotations (such as int n) do not exist; they are effectively thrown away. So at runtime, the following statement int x = 1 is equivalent to var x = 1.

A binding x is created but the type annotation is not used.

With the checked mode, Dart helps you catch type errors during development. This is in contrast to the other dynamic languages, such as Python, Ruby, and JavaScript, where these are only caught during testing, or much worse, they provoke runtime exceptions. You can easily check whether your Dart app runs in the checked mode or not by calling the function isCheckedMode() from main() (see the script test_checked_mode\bin\ test_checked_mode.dart in the Chapter 1 folder of the code bundle), as shown in the following code:

main() {
  isCheckedMode();
  // your code starts here  
}

void isCheckedMode() {
    try {
        int n = '';
        throw new Exception("Checked Mode is disabled!");
    } on TypeError {
      print("Checked Mode is enabled!");
    }
}

The exception message will be shown in the browser console. Be sure to remove this call or comment it out before deploying it to the production mode; we don't want an exception at runtime!