dart2js: Dart-to-JavaScript compiler

The dart2js tool provides hints for improving your Dart code and removing unused code. Also see which performs a similar analysis but 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.

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

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.

Note: The -O*n* argument specifies the optimization level. We recommend starting at -O1 (the default) and then increasing to -O2 or higher when you’re ready to deploy. The -O3 and -O4 optimization levels are suitable only for well tested code (see the -O*n* descriptions, below).

You can also configure dart2js options in a build config file. For more information, see the

Basic options

Common command-line options for dart2js include:

-o <file> or --out=<file>

Generates the output into <file>. If not specified, the output goes in a file named out.js.

--enable-asserts

Enables assertion checking.

-O{0|1|2|3|4}

Controls optimizations that can help reduce code size and improve performance of the generated code. For more details on these optimizations, run dart2js -hv.

  • -O0: Disables many optimizations.
  • : Enables default optimizations.

  • -O2: Enables -O1 optimizations, plus additional ones (such as minification) that respect the language semantics and are safe for all programs.

    Note: With -O2, string representations of types are no longer the same as those in the Dart VM and .

  • -O3: Enables -O2 optimizations, plus omits implicit type checks.

    Warning: Omitting type checks can cause your app to crash due to type errors. Before using -O3, test using -O2 to ensure that your app never throws a subtype of Error (such as TypeError).

  • -O4: Enables more aggressive optimizations than -O3, but with the same assumptions.

-h or

Displays help. To get information about all options, use -hv.

Path and environment options

Some other handy options include:

--packages=<path>

Specifies the path to the package resolution configuration file. For more information, see

-D<flag>=<value>

Defines an environment variable.

--version

Displays version information for dart2js.

Display options

The following options help you control the output of dart2js:

--suppress-warnings

Doesn’t display warnings.

--suppress-hints

Doesn’t display hints.

--terse

Emits diagnostics, without suggesting how to get rid of the diagnosed problems.

-v or --verbose

Displays lots of information.

Analysis options

--enable-diagnostic-colors

Adds colors to diagnostic messages.

--show-package-warnings

Shows warnings and hints generated from packages.

--csp

Disables dynamic generation of code in the generated output. This is necessary to satisfy CSP restrictions (see W3C Content Security Policy.)

--dump-info

Generates a file (with the suffix .info.json) that contains information about the generated code. You can inspect the generated file with the

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

  • Don’t use .
  • Don’t override noSuchMethod().
  • Avoid setting variables to null.

Note: Don’t worry about the size of your app’s included libraries. The dart2js tool performs tree shaking to omit unused classes, functions, methods, and so on. Just import the libraries you need, and let dart2js get rid of what you don’t need.

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: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
  3. Enable debugging, either on all exceptions or only on uncaught exceptions, as described in How to set breakpoints.
  4. Reload your app.

To debug in Edge:

  1. Update to the latest version of Edge.
  2. Load Developer Tools (F12). For more information, see
  3. Reload the app. 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, .

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

  1. Reload your app.
  1. Turn on the Develop menu, as described in the Safari Web Inspector Tutorial.
  2. Reload your app.