TextCodec

Pure WAT polyfill for TextEncoder/TextDecoder leveraging the wasm js-string builtin to provide fast conversion.

I built this to support WASM in isolated-vm projects, which do not and can not access native TextEncoder/TextDecoder.

The specific use case was supporting Rust wasm_bindgen projects.

Usage

Building WAT file will produce a WASM module you can load:

const wasmModule = new WebAssembly.Module(wasmBytes, {
		builtins: ["js-string"]
});

Methods exported from src/codec.wat

uint16array_to_string

Convert a region of WASM linear memory (interpreted as a Uint16Array of UTF-16 code units) to a JavaScript string. This is a fast path to get a string from character codes.

string_to_gcarray

Converts a javascript string to a wasm gc array.

string_to_utf8

Provides our encodeInto behavior. Converts a javascript string to UTF-8, populating a Uint8Array slice.

utf8_to_string

Converts a UTF-8 string (stored in a Uint8Array slice) into a javascript string. Provides our decode functionality.

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Stuff I might add later

• Streaming support: Functions that maintain state for incremental encoding/decoding
• Support for fatal/non-fatal decoding
• Support for BOM handling
• Support for other encodings (optional, rarely needed