oxc_coverage_instrument

Istanbul-compatible JavaScript/TypeScript coverage instrumentation, built on the Oxc parser. 5-33x faster than existing tools.

Why

swc-coverage-instrument fills this role for SWC. There is no equivalent for the Oxc ecosystem. Any tool built on oxc_parser that needs coverage instrumentation currently has to pull in SWC or Babel.

This crate fills that gap. AST-level instrumentation via oxc_traverse + oxc_codegen produces correct Istanbul-compatible output, verified against the canonical istanbul-lib-instrument on 25 shared fixtures.

Install

Rust

[dependencies]
oxc_coverage_instrument = "0.2"

Node.js

npm install oxc-coverage-instrument

CLI

cargo install oxc-coverage-instrument-cli

Usage

Rust

use oxc_coverage_instrument::{instrument, InstrumentOptions};

let source = "function add(a, b) { return a + b; }";
let result = instrument(source, "add.js", &InstrumentOptions::default()).unwrap();

// Istanbul-compatible coverage map
assert_eq!(result.coverage_map.fn_map["0"].name, "add");

// Instrumented source with counters injected
println!("{}", result.code);

Node.js

import { instrument } from 'oxc-coverage-instrument';

const result = instrument(source, 'file.js', {
  coverageVariable: '__coverage__',  // optional
  sourceMap: true,                    // optional
});

result.code;                          // instrumented source
const coverageMap = JSON.parse(result.coverageMap);  // Istanbul format
result.sourceMap;                     // source map JSON (if enabled)

CLI

# Print instrumented code to stdout
oxc-coverage-instrument src/app.js

# Write to file
oxc-coverage-instrument src/app.js -o dist/app.js

# Print coverage map only
oxc-coverage-instrument src/app.js --coverage-map

# With source map
oxc-coverage-instrument src/app.js -o dist/app.js --source-map

Vitest integration

import { defineConfig } from 'vitest/config'
import { createOxcInstrumenter } from 'oxc-coverage-instrument/vitest'

export default defineConfig({
  test: {
    coverage: {
      provider: 'istanbul',
      instrumenter: () => createOxcInstrumenter(),
    }
  }
})

Note: Requires Vitest with custom instrumenter support (see vitest#10119).

Vite plugin example

import { instrument } from 'oxc-coverage-instrument';

export function coveragePlugin() {
  return {
    name: 'coverage-instrument',
    transform(code, id) {
      if (process.env.COVERAGE && /\.[jt]sx?$/.test(id) && !id.includes('node_modules')) {
        const result = instrument(code, id, { sourceMap: true });
        return { code: result.code, map: result.sourceMap ? JSON.parse(result.sourceMap) : undefined };
      }
    },
  };
}

Reading existing coverage data

use oxc_coverage_instrument::parse_coverage_map;

// Parse a coverage-final.json file
let json = std::fs::read_to_string("coverage-final.json").unwrap();
let map = parse_coverage_map(&json).unwrap();

for (path, coverage) in &map {
    println!("{}: {} statements, {} functions, {} branches",
        path, coverage.s.len(), coverage.f.len(), coverage.b.len());
}

What it tracks

Istanbul conformance

Verified against istanbul-lib-instrument on 25 shared fixtures covering all branch types, function forms, and edge cases. 175 automated conformance checks validate:

• Function counts and names match exactly
• Branch counts, types, and location counts match exactly
• Statement counts within tolerance
• JSON structure matches Istanbul's field set
• Instrumented output re-parses as valid JS

Performance

Benchmarked on real-world JavaScript libraries, all running in the same Node.js process for a fair comparison. Reproduce with ./scripts/benchmark-comparison.sh.

8-13x faster than babel-plugin-istanbul, 10-16x faster than swc-plugin-coverage-instrument (Rust/WASM), 30-48x faster than istanbul-lib-instrument.

Note: swc-plugin-coverage-instrument is written in Rust but runs as a WASM module inside SWC's sandbox, adding serialisation overhead at every AST boundary. The comparison measures end-to-end instrumentation time as users experience it.

Architecture

source code (JS/TS)
    |
    v
oxc_parser          -- parse to AST
    |
    v
SemanticBuilder     -- build scope tree
    |
    v
CoverageTransform   -- traverse AST, inject ++cov().s[N] counters
    |
    v
oxc_codegen         -- emit instrumented code + source map
    |
    v
instrumented code + coverage map

Related projects

Compatibility

• Rust: 1.92+ (2024 edition)
• Oxc: 0.124.x
• Istanbul: coverage-final.json v3+ format
• Node.js: 18+ (via napi-rs)

License

MIT