This page answers common questions about the Dart dev compiler (dartdevc). For information on how to use dartdevc, see the dartdevc documentation.

The basics

What is dartdevc?

The dartdevc tool is a Dart-to-JavaScript compiler that’s targeted at web app development in modern browsers. The existing Dart-to-JavaScript compiler, dart2js, is still supported.

How do I use dartdevc?

Don’t run dartdevc directly. The webdev and build_runner tools use dartdevc as the default web compiler when building, serving, and testing web apps.

When should I use dartdevc?

Use dartdevc whenever you’re actively working on your code. Here are some of the advantages of dartdevc over dart2js:

• Faster refreshing after code changes.
• Simpler, more readable, more idiomatic JavaScript output containing fewer runtime checks.
• Integration with Chrome dev tools for easier debugging.

Keep using dart2js to build your deployed, production app. With dart2js you get advanced optimizations such as tree shaking to minimize downloaded code size.

What browsers does dartdevc work with?

Chrome is the only supported browser. You might be able to use other modern browsers (Edge, Firefox, and Safari).

If you find a bug in dartdevc’s support for Chrome, Edge, Firefox, or Safari, please create an issue.

Common problems

My code analyzes correctly, so why am I getting compile-time errors?

Because dartdevc analyzes more code.

The Dart analyzer looks at an individual package’s source code, not the code of any additional packages. In contrast, dartdevc analyzes the whole, generated program; it covers the original source code, plus generated code and all used packages.

My code analyzes correctly, so why am I getting runtime errors?

Some of Dart’s type safety checks are implemented only at runtime. Even if your code has no static type errors, you may see runtime errors.

Why does dartdevc take longer to run the first time?

Because it’s compiling your entire app, including the packages your app depends on.

The first time you build or serve your app, dartdevc compiles every module. Afterward, as long as webdev continues to run, it tracks which Dart files change, and dartdevc recompiles only the modules that are affected by those changes.

Why are subsequent runs taking a long time?

Are the implementation files for your package under lib/src? If not, dartdevc is probably creating too many modules. For more information, see How are the modules created?

Why is dartdevc producing so many JavaScript files?

This happens when dartdevc is creating too many modules. Are the implementation files for your package under lib/src? For more information, see How are the modules created?

Why doesn’t dartdevc lazily load deferred libraries?

Lazily loading libraries is a production use case, and dartdevc isn’t intended for production code. However, dartdevc validates that when code uses a deferred library, that code first calls loadLibrary().

Where can I see known problems with dartdevc?

Issues are in the Dart SDK repo with the labels area-web and web-dev-compiler.

Modules

What are JavaScript modules?

When you use dartdevc, modules are an implementation detail.

The dartdevc creates several JavaScript modules, each of which contains code generated from one or more Dart files. When you edit your Dart files, webdev recompiles only the affected modules, instead of your whole app. The result is a much quicker edit-refresh cycle.

By contrast, when you use dart2js, dart2js creates one JavaScript file for the entire app.

If you’d like to know more about JavaScript modules, see Preethi Kasireddy’s beginner’s guide or Addy Osmani’s article on writing modular JavaScript.

How are the modules created?

When you use a webdev command with dartdevc, a heuristic that’s based on package structure determines which modules the webdev command creates:

• One module for each Dart file that’s under lib, but not under lib/src.
  These are the Dart files that are part of the package’s public API, and can be imported by other packages.

• One module for each Dart file that’s not under lib and that contains a top-level main() function.
  For example, web/main.dart gets its own module.

• One module for each Dart file that’s not imported by one of the above.

Any Dart file that is imported ends up either directly in the importing file’s module or (if it’s imported more than once) in a shared module.

These rules ensure that no Dart file is compiled into more than one module. Importing only from lib and not lib/src minimizes the amount of code that your app loads.

What kind of modules does dartdevc produce?

When run with webdev, dartdevc generates AMD modules. Dartdevc can also generate ES6 (Harmony) and CommonJS (Node) modules, but these options aren’t currently supported by the build_web_compilers package used with the build_runner and webdev tools.

Can I customize my modules?

Not today, but possibly in the future.

Resources

Where can I talk to people who are using dartdevc?

Try one of the Dart Gitter rooms.

Where can I learn what to do about common dartdevc errors?

Although you can find help for common analyzer errors and warnings, there’s no similar help for dartdevc-specific errors. See SDK issue #29825 for more information.

Where can I learn the gory details of dartdevc?

You can find source code and more documentation in the Dart SDK repo under /pkg/dev_compiler.