wasmer/lib/js-api/README.md

wasmer-js

Wasmer is the most popular WebAssembly runtime for Rust. This crate mimics the same Rust API than the wasmer crate, but when compiled to WebAssembly, it only targets a JavaScript host. It means that it is possible to write a Rust program that uses Wasmer, and compiles everything to WebAssembly to run in a browser, Node.js, Deno and so on.

Usage

We recommend aliasing wasmer_js to wasmer at the top of your crate.

#[cfg(feature = "js")]
extern crate wasmer_js as wasmer;

And then:

use wasmer::{Store, Module, Instance, Value, imports};

#[wasm_bindgen]
pub extern fn do_add_one_in_wasmer() -> i32 {
    let module_wat = r#"
    (module
    (type $t0 (func (param i32) (result i32)))
    (func $add_one (export "add_one") (type $t0) (param $p0 i32) (result i32)
        get_local $p0
        i32.const 1
        i32.add))
    "#;

    let store = Store::default();
    let module = Module::new(&store, &module_wat).unwrap();
    // The module doesn't import anything, so we create an empty import object.
    let import_object = imports! {};
    let instance = Instance::new(&module, &import_object).unwrap();

    let add_one = instance.exports.get_function("add_one").unwrap();
    let result = add_one.call(&[Value::I32(42)]).unwrap();
    assert_eq!(result[0], Value::I32(43));
    result[0].unwrap_i32()
}

Feature flags

wasmer-js has the following feature flags:

• wasm-types-polyfill (enabled by default): it parses the Wasm file, allowing to do type reflection of the inner WebAssembly types. It adds 100Kb to the Wasm bundle (28Kb gzipped). You can disable it and use Module::set_type_hints manually instead if you want a lightweight alternative. This is needed until the Wasm JS introspection API proposal is adopted by browsers

• wat: It allows to read WebAssembly files in their text format. This feature is normally used only in development environments, it will add around 650Kb to the Wasm bundle (120Kb gzipped).

Build

You can use wasm-pack to build wasmer-js-api:

wasm-pack build --release

The provided wasmer_js.wasm file should weight around 60kB (27Kb gzipped) when optmized via wasm-opt and stripped via wasm-strip, so it's quite slim.

Test

wasm-pack test --node

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Made with ❤️ by the Wasmer team, for the community