dart2js: The Dart-to-JavaScript Compiler

Use the dart2js tool to compile Dart code to JavaScript. Dart Editor uses dart2js behind the scenes whenever Dart Editor compiles to JavaScript. The pub serve, pub run, and pub build commands also use dart2js. If you are using dart2js through one of the pub commands, see Configuring the Built-in dart2js Transformer for Pub for information on how to specify dart2js flags in your pubspec file.

The dart2js tool provides hints for improving your Dart code and removing unused code. You can get these hints for all kinds of code—even command-line apps. Also see dartanalyzer, which performs a similar analysis but, as of 1.0, has a different implementation.

This page tells you how to use dart2js on the command line. It also give tips on debugging the JavaScript that dart2js generates.

Basic usage

Here’s an example of compiling a Dart file to JavaScript:

dart2js --out=test.js test.dart

This command produces a file that contains the JavaScript equivalent of your Dart code. It also produces a source map, which can help you debug the JavaScript version of the app more easily.

Usage in pubspec

You can also configure dart2js options in the pubspec file. For more information, see Configuring the Built-in dart2js Transformer for Pub.


Common command-line options for dart2js include:

-o <file> or --out=<file>
Generate the output into <file>. If not specified, the output goes in a file named out.js.
-c or --checked
Insert runtime type checks, and enable assertions (checked mode).
-m or --minify
Generate minified output.
-h or --help
Display help. (Use -vh for information about all options.)

Some other handy options include:

-p <path> or --package-root=<path>
Specify where to find “package:” imports.
Define an environment variable.
Display version information for dart2js.

The following options help you control the output of dart2js:

Don’t display warnings.
Don’t display hints.
Emit diagnostics, but don’t suggest how to get rid of the diagnosed problems.
-v or --verbose
Display lots of information.

The following options control the analysis that dart2js performs on Dart code:

Analyze even the code that isn’t reachable from main(). This option is useful for finding errors in libraries, but using it can result in bigger and slower output.
Analyze the code, but don’t generate code.
Like --analyze-only, but skip analysis of method bodies and field initializers.
Add colors to diagnostic messages.
Show warnings and hints generated from packages.
If true, disables dynamic generation of code in the generated output. This is necessary to satisfy CSP restrictions (see W3C Content Security Policy). The default is false.
Use with --analyze-only to analyze a command-line app. The default category is Client, which tells dart2js to expect a web app. This option is only for dart2dart.
Generates an out.info.json file with information about the generated code. You can inspect the generated file with the viewer.
Output Dart code instead of JavaScript. If you are looking for dart2dart, use this option.

Helping dart2js generate better code

You can do a couple of things to improve the code that dart2js generates:

  • Write your code in a way that makes type inference easier.

  • Once you’re ready to deploy your app, use the dart2js --minify option to reduce code size.

Follow these practices to help dart2js do better type inference, so it can generate smaller and faster JavaScript code:

  • Avoid using the dart:mirrors library, directly or indirectly. If you must use it, provide @MirrorsUsed annotations.

  • Don’t use Function.apply().

  • Don’t override noSuchMethod().

  • Avoid setting variables to null.

  • Be consistent with the types of arguments you pass into each function or method.


This section gives tips for debugging dart2js-produced code in Chrome, Firefox, and Safari. Debugging the JavaScript produced by dart2js is easiest in browsers such as Chrome that support source maps.

Whichever browser you use, you should enable pausing on at least uncaught exceptions, and perhaps on all exceptions. For frameworks such as dart:isolate and dart:async that wrap user code in try-catch, we recommend pausing on all exceptions.


To debug in Chrome:

  1. Open the Developer Tools window, as described in the Chrome DevTools documentation.

  2. Turn on source maps, as described in the video SourceMaps in Chrome.

  3. Enable debugging, either on all exceptions or only on uncaught exceptions, as described in Pause on Uncaught Exceptions.

  4. Reload your application.

Internet Explorer

To debug in Internet Explorer:

  1. Update to the latest version of Internet Explorer. (Source-map support was added to IE in April 2014).

  2. Load Developer Tools (F12). For more information, see Using the F12 developer tools.

  3. Reload the application. The debugger tab shows source-mapped files.

  4. Exception behavior can be controlled through Ctrl+Shift+E; the default is Break on unhandled exceptions.


Firefox doesn’t yet support source maps (see bug #771597).

To debug in Firefox:

  1. Enable the Developer Toolbar, as described in Kevin Dangoor’s blog post, New Firefox Command Line Helps You Develop Faster".

  2. Click Pause on exceptions, as shown in the following figure.
  3. Firefox Toolbar

  4. Reload your application.


To debug in Safari:

  1. Turn on the Develop menu, as described in the Safari Web Inspector Guide.

  2. Enable breaks, either on all exceptions or only on uncaught exceptions. See Figure 4-2 on the Safari Debugging page.

  3. Reload your application.