WebAssembly (WASM) Build
There is already a fast and powerful scripting language that integrates nicely with WASM – JavaScript.
Anyhow, do it because you can!
It is possible to use Rhai when compiling to WebAssembly (WASM).
This yields a scripting engine (and language) that can be run in a standard web browser, among other places.
Check out the Online Playground project which is driven
by a Rhai Engine compiled into WASM.
JavaScript Interop
Specify either of the wasm-bindgen or stdweb features when building for WASM that requires
interop with JavaScript. This selects the appropriate JavaScript interop layer to use.
It is still possible to compile for WASM without either wasm-bindgen or stdweb,
but then the interop code must then be explicitly provided.
Target Environments
Rhai requires a system-provided source of random numbers (for hashing).
Such random number source is available from JavaScript (implied by wasm-bindgen or stdweb).
The js feature on the getrandom crate is
enabled automatically to provide the random number source.
See also: https://docs.rs/getrandom/latest/getrandom/#webassembly-support for details.
Rhai requires a system-provided source of random numbers (for hashing).
Non-JavaScript/non-browser environments may not have random numbers available, so it is necessary to
opt out of default-features in order to enable static hashing which uses fixed (non-random) keys.
[dependencies]
rhai = { version = "1.18.0", default-features = false, features = [ "std" ] }
Size
Also look into minimal builds to reduce generated WASM size.
A typical, full-featured Rhai scripting engine compiles to a single WASM32 file that is less than 400KB (non-gzipped).
When excluding features that are marginal in WASM environment, the gzipped payload can be shrunk further.
Standard packages can also be excluded to yield additional size savings.
Speed
In benchmark tests, a WASM build runs scripts roughly 30% slower than a native optimized release build.
Common Features
Some Rhai functionalities are not necessary in a WASM environment, so the following features are typically used for a WASM build:
| Feature | Description |
|---|---|
wasm-bindgen or stdweb | use wasm-bindgen or stdweb as the JavaScript interop layer, omit if using custom interop code |
unchecked | when a WASM module panics, it doesn’t crash the entire web app; however this also disables maximum number of operations and progress tracking so a script can still run indefinitely – the web app must terminate it itself |
only_i32 | WASM supports 32-bit and 64-bit integers, but most scripts will only need 32-bit |
f32_float | WASM supports 32-bit single-precision and 64-bit double-precision floating-point numbers, but single-precision is usually fine for most uses |
no_module | a WASM module cannot load modules from the file system, so usually this is not needed, but the savings are minimal; alternatively, a custom module resolver can be provided that loads other Rhai scripts |
no_custom_syntax | if custom syntax is not used, this results in a small size saving |
The following features are typically not used because they don’t make sense in a WASM build: