Running dart2wasm

You don't need to build the Dart SDK to run dart2wasm, as long as you have a Dart SDK installed.

To compile a Dart file to Wasm, run:

dart --enable-asserts pkg/dart2wasm/bin/dart2wasm.dart options infile.dart outfile.wasm

where options include:

--dart-sdk=pathrelative to scriptThe location of the sdk directory inside the Dart SDK, containing the core library sources.
--platform=pathnoneThe location of the platform dill file containing the compiled core libraries.
--[no-]export-allnoExport all functions; otherwise, just export main.
--[no-]import-shared-memorynoImport a shared memory buffer. If this is on, --shared-memory-max-pages must also be specified.
--[no-]inliningnoInline small functions.
--[no-]lazy-constantsnoInstantiate constants lazily.
--[no-]local-nullabilitynoUse non-nullable types for non-nullable locals and temporaries.
--[no-]name-sectionyesEmit Name Section with function names.
--[no-]nominal-typesyesEmit nominal types.
--[no-]parameter-nullabilityyesUse non-nullable types for non-nullable parameters and return values.
--[no-]polymorphic-specializationnoDo virtual calls by switching on the class ID instead of using call_indirect.
--[no-]print-kernelnoPrint IR for each function before compiling it.
--[no-]print-wasmnoPrint Wasm instructions of each compiled function.
--[no-]runtime-typesnoUse RTTs for allocations and casts.
--shared-memory-max-pages pagecountMax size of the imported memory buffer. If --shared-import-memory is specified, this must also be specified.
--[no-]string-data-segmentsnoUse experimental array init from data segment for string constants.
--watch offsetPrint stack trace leading to the byte at offset offset in the .wasm output file. Can be specified multiple times.

The resulting .wasm file can be run with:

d8 --experimental-wasm-gc --wasm-gc-js-interop pkg/dart2wasm/bin/run_wasm.js -- outfile.wasm

Imports and exports

To import a function, declare it as a global, external function and mark it with a wasm:import pragma indicating the imported name (which must be two identifiers separated by a dot):

@pragma("wasm:import", "")
external void fooBar(Object object);

which will call on the host side:

var foo = {
    bar: function(object) { /* implementation here */ }

To export a function, mark it with a wasm:export pragma:

void foo(double x) { /* implementation here */  }

@pragma("wasm:export", "baz")
void bar(double x) { /* implementation here */  }

With the Wasm module instance in inst, these can be called as:;

Types to use for interop

In the signatures of imported and exported functions, use the following types:

  • For numbers, use double.
  • For Dart objects, use the corresponding Dart type. The fields of the underlying representation can be accessed on the JS side as .$field0, .$field1 etc., but there is currently no defined way of finding the field index of a particular Dart field, so this mechanism is mainly useful for special objects with known layout.
  • For JS objects, use the WasmAnyRef type (or WasmAnyRef? as applicable) from the dart:wasm package. These can be passed around and stored as opaque values on the Dart side.