Questions? Suggestions? Found a bug? Please file an issue or start a discussion.

Web compilers for build_runner.

Installation

This package is intended to be used as a development dependency for users of package:build who want to run code in a browser. Simply add the following to your pubspec.yaml:

dev_dependencies:
  build_web_compilers:

Usage

If you are using the autogenerated build script (going through dart run build_runner <command> instead of handwriting a build.dart file), then all you need is the dev_dependency listed above.

Configuration

By default, this package uses the Dart development compiler (dartdevc, also known as DDC) to compile Dart to JavaScript. In release builds (running the build tool with --release), dart2js is used as a compiler.

In addition to compiling to JavaScript, this package also supports compiling to WebAssembly. Currently, dart2wasm needs to be enabled with builder options. To understand the impact of these options, be aware of differences between compiling to JavaScript and compiling to WebAssembly:

  1. Dart has two compilers emitting JavaScript: dart2js and dartdevc (which supports incremental rebuilds but is typically only used for development). For both JavaScript compilers, build_web_compilers generates a primary entrypoint script and additional module files or source maps depending on compiler options.
  2. Compiling with dart2wasm generates a WebAssembly module (a .wasm file) and a JavaScript module (a .mjs file) exporting functions to instantiate that module. dart2wasm alone generates no entrypoint file that could be added as a <script> tag.

In addition to invoking compilers, build_web_compilers can emit an entrypoint file suitable for dart2wasm. When both dart2wasm and a JavaScript compiler are enabled, the entrypoint file runs a feature detection for WasmGC and loads either the WebAssembly module or the JavaScript file depending on what the browser supports.

Compiler arguments

To customize compilers, you will need to add a build.yaml file configuring the build_web_compilers:entrypoint builder, similar to the following:

targets:
  $default:
    builders:
      build_web_compilers:entrypoint:
        # These are globs for the entrypoints you want to compile.
        generate_for:
        - test/**.browser_test.dart
        - web/**.dart
        options:
          compilers:
            # All compilers listed here are enabled:
            dart2js:
              # List any dart2js specific args here, or omit it.
              args:
                - O2
            dart2wasm:
              args:
                - O3

build_runner runs development builds by default, but can use builders with a different configuration for --release builds. For instance, if you wanted to compile with dartdevc only on development and dart2js + dart2wasm for release builds, you can use this configuration as a starting point:

targets:
  $default:
    builders:
      build_web_compilers:entrypoint:
        generate_for:
        - test/**.browser_test.dart
        - web/**.dart
        dev_options:
          compilers:
            dartdevc:
        release_options:
          compilers:
            dart2js:
              args:
                - O2
            dart2wasm:
              args:
                - O3

Customizing emitted file names

The file names emitted by build_web_compilers can be changed. The default names depend on which compilers are enabled:

  1. When only dart2js or dartdevc is enabled, a single .dart.js entrypoint is emitted.
  2. When only dart2wasm is enabled, a single .dart.js entrypoint (loading a generated .wasm module through a .mjs helper file) is generated.
  3. When both dart2wasm and a JavaScript compiler are enabled, then:
    • The output of the JavaScript compiler is named .dart2js.js or .ddc.js depending on the compiler.
    • dart2wasm continues to emit a .wasm and a .mjs file.
    • An entrypoint loader named .dart.js that loads the appropriate output depending on browser features is generated.

All names can be overridden by using the loader option or the extension flag in compiler options:

targets:
  $default:
    builders:
      build_web_compilers:entrypoint:
        options:
          loader: .load.js
          compilers:
            dart2js:
              extension: .old.js
            dart2wasm:
              extension: .new.mjs

This configuration uses both dart2js and dart2wasm, but names the final entrypoint for a main.dart file main.load.js. That loader will either load a .new.mjs file for WebAssembly or a .old.js for pure JavaScript.

Note that the loader option is ignored when dart2wasm is not enabled, as it's the compiler requiring an additional loader to be emitted.

Customizing the WebAssembly loader

In some cases, for instance when targeting Node.JS, the generated loader for dart2wasm may be unsuitable. The builtin loader can be disabled by setting the option to null:

targets:
  $default:
    builders:
      build_web_compilers:entrypoint:
        options:
          loader: null
          compilers:
            dart2js:
            dart2wasm:

In this case, you need to use another builder or a predefined loader instead.

Configuring -D environment variables

dartdevc is a modular compiler, so in order to ensure consistent builds in every module environment variables must be configured globally. Configure with a mapping in YAML. Environment defined variables can be read with const String.fromEnvironment and const bool.fromEnvironment. For example:

global_options:
  build_web_compilers:ddc:
    options:
      environment:
        SOME_VAR: some value
        ANOTHER_VAR: false

These may also be specified on the command line with a --define argument.

webdev serve -- '--define=build_web_compilers:ddc=environment={"SOME_VAR":"changed"}'

For dart2js, use the args option within the dart2js compiler entry. This may be configured globally, or per target.

targets:
  $default:
    builders:
      build_web_compilers:entrypoint:
        options:
          compilers:
            dart2js:
              args:
                - -DSOME_VAR=some value
                - -DANOTHER_VAR=true

To apply variables across multiple compilers, they have to be added to each one:

targets:
  $default:
    builders:
      build_web_compilers:entrypoint:
        options:
          compilers:
            dart2js:
              args:
                - -DSOME_VAR=some value
                - -DANOTHER_VAR=true
            dart2wasm:
              args:
                - -DSOME_VAR=some value
                - -DANOTHER_VAR=true

Libraries

build_web_compilers
builders