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Building to WebAssembly (WASM)
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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.
Why you would want to is another matter... as there is already a nice, fast, complete scripting language for the the common WASM environment (i.e. a browser) - and it is called JavaScript.
But anyhow, do it because you can!
When building for WASM, certain features will not be available, such as the script file API's and loading modules from external script files.
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 200KB gzipped.
When excluding features that are marginal in WASM environment, the gzipped payload can be further shrunk to 160KB.
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 |
|---|---|
[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] |
JavaScript has only one number type and we're only supporting wasm32 here (so far). |
[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:
| Feature | Why unnecessary |
|---|---|
[sync] |
WASM is single-threaded. |
[no_std] |
std lib works fine with WASM. |
[internals] |
WASM usually doesn't need to access Rhai internal data structures. |