@create-font/fea-rs-wasm

WebAssembly bindings for fea-rs and the create-font Adobe Feature File (.fea) formatter, packaged for Node and browsers.

fea-rs owns the Adobe feature grammar, parser, error recovery, and lossless concrete syntax tree. This package does not implement a separate parser or AST. It provides:

• a versioned JSON projection of the fea-rs concrete syntax tree and diagnostics;
• generated WebAssembly entry points for browsers and Node;
• the create-font deterministic, idempotent formatter.

JavaScript API

Use the browser entry point and initialize it before calling the synchronous exports:

import init, { formatFea, parseFea } from "@create-font/fea-rs-wasm/web"

await init()
const syntax = JSON.parse(parseFea("feature liga { sub f i by f_i; } liga;"))
const formatted = formatFea(
	"feature liga{sub f i by f_i;}liga;",
	JSON.stringify({ lineWidth: 100 }),
)

The Node entry point initializes itself:

import { formatFea, parseFea } from "@create-font/fea-rs-wasm/node"

parseFea invokes fea-rs and returns JSON with abiVersion, sourceLen, a recursive root, and recoverable diagnostics. Every node and token has a half-open UTF-8 byte range. Tokens include their exact source text, including whitespace and comments, so concatenating the token text reconstructs the input byte for byte.

ABI version 1 uses owned JavaScript strings on both sides of each call. The module owns its linear memory, and the Rust API keeps no document handles, cached trees, or source text after a call returns. A breaking JSON shape, diagnostic code, or ownership change requires an ABI version bump.

Includes are intentionally not loaded by the binding boundary. An include(path.fea) statement remains in the concrete syntax tree; the host owns filesystem or network resolution.

formatFea accepts JSON with these properties:

• lineWidth (default 80);
• indentWidth (default 2);
• useTabs (default false);
• newLineKind: "auto", "lineFeed", or "carriageReturnLineFeed".

Formatting is fail-closed. If parsing reports an error, no rewritten text is returned.

Syntax coverage and deviations

Parsing is provided by fea-rs 0.22 and follows its Adobe Feature File grammar, including GSUB and GPOS rules, named lookups, glyph classes, value records, feature variations, and the supported Adobe table blocks. The repository fixture exercises languagesystems, classes, includes, lookup blocks, substitution, contextual rules, positioning, value records, and GDEF.

This binding intentionally exposes syntax parsing only. Project-aware include loading, glyph inventory checks, lookup validation, and create-font semantic lowering belong to the host analysis layer. Diagnostic codes and byte ranges are normalized at the binding boundary; explanatory text and syntax-kind names follow the pinned fea-rs release.

Rebuilding

Install the versions pinned by the repository and build:

mise install
pnpm --filter @create-font/fea-rs-wasm build

The build uses mise-managed Cargo, Rust, the wasm32-unknown-unknown target, and wasm-bindgen-cli. Generated bindings and Wasm stay under the ignored dist/ directory and are included only in the published package.

Run the package checks with:

pnpm --filter @create-font/fea-rs-wasm build
pnpm --filter @create-font/fea-rs-wasm test
pnpm --filter @create-font/fea-rs-wasm pack --dry-run

dist/create-font-fea-rs-wasm.json records the ABI version, byte size, and SHA-256 of both runtime artifacts.

Reference size and performance

A release runtime built with Rust 1.97.1 is approximately 278 kB. Regenerate performance measurements after a build with:

pnpm --filter @create-font/fea-rs-wasm benchmark