In this use-case we wish to use a JavaScript program to invoke a WebAssembly module that further invokes native "OS" functionality. In other words, we wish to implement the following call chain:
JavaScript has native support for WebAssembly, but without a tool such as the WebAssembly System Interface (WASI), a WebAssembly module does not normally have access to any OS-level functionality.
Within the context of a JavaScript runtime environment, WASI functionality must be provided by means of a set of JavaScript polyfills — and this is the role of Wasmer-js.


The JavaScript environment shown here could either be on the client (within a browser), or on the server (provided by Node.js). Either way, the native functions invoked from WebAssembly via Wasmer-JS and WASI belong to the JavaScript runtime environment, not the underlying operating system.