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.
When building for WASM, certain features will not be available,
such as the script file API's and loading [modules] from external script files.
Check out the [_Online Playground_]({{rootUrl}}/tools/playground.md) 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.
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 be explicitly provided.
Size
Also look into minimal builds to reduce generated WASM size.
As of this version, a typical, full-featured Rhai scripting engine compiles to a single WASM file less than 300KB gzipped.
When excluding features that are marginal in WASM environment, the gzipped payload can be further shrunk to 160KB.
The following standard packages, when excluded, provide the corresponding size savings:
| Package | WASM size saving |
|---|---|
CorePackage | 18 KB |
BitFieldPackage | 1 KB |
LogicPackage | < 1 KB |
BasicMathPackage | 1 KB |
BasicArrayPackage | 15 KB |
BasicBlobPackage | 7 KB |
BasicMapPackage | 4 KB |
MoreStringPackage | 34 KB |
Speed
In benchmark tests, a WASM build runs scripts roughly 1.7-2.2x 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 |
The following features are typically not used because they don’t make sense in a WASM build: