Dynamically linked modules

An important use for Javy is for when you may want or need to generate much smaller Wasm modules. Using the -C dynamic flag when invoking javy build will create a dynamically linked module which will have a much smaller file size than a statically linked module. Statically linked modules embed the JS engine inside the module while dynamically linked modules rely on Wasm imports to provide the JS engine. Dynamically linked modules have special requirements that statically linked modules do not and will not execute in WebAssembly runtimes that do not meet these requirements.

To successfully instantiate and run a dynamically linked Javy module, the execution environment must provide a collection of imports that match the exports from a Javy plugin Wasm module (for example, cabi_realloc and invoke). The namespace for these imports must match the import namespace specified by the Javy plugin Wasm module used to generate the dynamically linked Wasm module (this is, if the plugin’s import namespace was my_plugin_v1, then the imports must be made available under the module name my_plugin_v1). This value is available from the import_namespace custom section in the Javy plugin module. You can also statically inspect the imports of the dynamically linked Wasm module to determine the import namespace. Dynamically linked modules cannot be instantiated in environments that do not provide the required imports.

Dynamically linked Javy modules are tied to QuickJS since they use QuickJS’s bytecode representation.

Obtaining the default plugin module

The plugin.wasm module is available as an asset on the Javy release you are using.

It can also be obtained by running javy emit-plugin -o <path> to write the module into <path>.

Creating and running a dynamically linked module through the CLI

Run:

$ echo 'console.log("hello world!");' > my_code.js
$ javy emit-plugin -o plugin.wasm
$ javy build -C dynamic -C plugin=plugin.wasm -o my_code.wasm my_code.js
$ wasmtime run --preload javy-default-plugin-v5=plugin.wasm my_code.wasm
hello world!