istanbul-lib-instrument 5.2.1: instrument JavaScript code using createInstrumenter and extract file coverage

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: instrument JavaScript code using createInstrumenter and extract file coverage
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/istanbul-lib-instrument@5.2.1
Demonstrate these symbols/APIs:
  - createInstrumenter
  - instrumentSync
  - lastFileCoverage

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

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{
  "name": "csx-sample-istanbul-lib-instrument",
  "version": "1.0.0",
  "private": true,
  "type": "module",
  "license": "MIT-0",
  "dependencies": {
    "istanbul-lib-instrument": "5.2.1"
  }
}

{
  "schemaVersion": 1,
  "goal": "instrument JavaScript code using createInstrumenter and extract file coverage",
  "kind": "HOW",
  "packages": [
    "pkg:npm/istanbul-lib-instrument@5.2.1"
  ],
  "symbols": [
    "createInstrumenter",
    "instrumentSync",
    "lastFileCoverage"
  ]
}

import { createInstrumenter, programVisitor, readInitialCoverage } from 'istanbul-lib-instrument';

export { createInstrumenter, programVisitor, readInitialCoverage };

/**
 * Instruments JavaScript source code and extracts file coverage metadata.
 *
 * @param {string} code - The JavaScript source code to instrument.
 * @param {string} filename - The file path / identifier associated with the code.
 * @param {object} [options={}] - Options passed to createInstrumenter.
 * @returns {{ code: string, fileCoverage: object, sourceMap: object|null }}
 */
export function instrumentCode(code, filename, options = {}) {
  const instrumenter = createInstrumenter({
    coverageVariable: '__coverage__',
    preserveComments: true,
    compact: false,
    esModules: true,
    produceSourceMap: true,
    ...options,
  });

  const instrumentedCode = instrumenter.instrumentSync(code, filename);
  const fileCoverage = instrumenter.lastFileCoverage();
  const sourceMap = instrumenter.lastSourceMap();

  return {
    code: instrumentedCode,
    fileCoverage,
    sourceMap,
  };
}

/**
 * Creates an instrumenter instance with customized coverage variable and settings.
 *
 * @param {object} [options={}] - Instrumenter options.
 * @returns {object} - Istanbul instrumenter instance.
 */
export function createCustomInstrumenter(options = {}) {
  return createInstrumenter({
    coverageVariable: '__coverage__',
    preserveComments: true,
    ...options,
  });
}

import { strict as assert } from 'node:assert';
import vm from 'node:vm';
import {
  createInstrumenter,
  programVisitor,
  readInitialCoverage,
  instrumentCode,
  createCustomInstrumenter,
} from '../src/index.mjs';

// 1. Verify module exports and types
assert.equal(typeof createInstrumenter, 'function', 'createInstrumenter must be a function');
assert.equal(typeof programVisitor, 'function', 'programVisitor must be a function');
assert.equal(typeof readInitialCoverage, 'function', 'readInitialCoverage must be a function');

// 2. Verify instrumenter instance creation and method availability
const instrumenter = createInstrumenter({
  coverageVariable: '__coverage__',
  preserveComments: true,
  produceSourceMap: true,
});
assert.equal(typeof instrumenter.instrumentSync, 'function', 'instrumenter must have instrumentSync method');
assert.equal(typeof instrumenter.lastFileCoverage, 'function', 'instrumenter must have lastFileCoverage method');
assert.equal(typeof instrumenter.lastSourceMap, 'function', 'instrumenter must have lastSourceMap method');

// 3. Instrument a sample JavaScript code snippet containing functions and branches
const filename = 'math.js';
const sampleCode = `
function add(a, b) {
  if (a > 0) {
    return a + b;
  }
  return b;
}
add(2, 3);
`;

const instrumented = instrumenter.instrumentSync(sampleCode, filename);
assert.equal(typeof instrumented, 'string', 'instrumentSync must return instrumented code string');
assert(instrumented.includes('__coverage__'), 'Instrumented code must reference coverage variable');
assert(instrumented.includes('path'), 'Instrumented code must contain coverage initialization');

// 4. Verify lastFileCoverage structure and metadata
const coverage = instrumenter.lastFileCoverage();
assert.ok(coverage, 'lastFileCoverage must return coverage data');
assert.equal(coverage.path, filename, 'Coverage path must match instrumented filename');
assert.ok(typeof coverage.statementMap === 'object', 'Coverage must have statementMap');
assert.ok(typeof coverage.fnMap === 'object', 'Coverage must have fnMap');
assert.ok(typeof coverage.branchMap === 'object', 'Coverage must have branchMap');
assert.ok(typeof coverage.s === 'object', 'Coverage must have statement hit counts');
assert.ok(typeof coverage.f === 'object', 'Coverage must have function hit counts');
assert.ok(typeof coverage.b === 'object', 'Coverage must have branch hit counts');

// Initial hit counters must be 0
for (const key of Object.keys(coverage.s)) {
  assert.equal(coverage.s[key], 0, `Statement counter ${key} must initially be 0`);
}
for (const key of Object.keys(coverage.f)) {
  assert.equal(coverage.f[key], 0, `Function counter ${key} must initially be 0`);
}
for (const key of Object.keys(coverage.b)) {
  for (const branchCount of coverage.b[key]) {
    assert.equal(branchCount, 0, `Branch counter ${key} must initially be 0`);
  }
}

// 5. Verify lastSourceMap generation
const sourceMap = instrumenter.lastSourceMap();
assert.ok(sourceMap, 'lastSourceMap must return a valid source map when produceSourceMap is true');
assert.equal(typeof sourceMap.version, 'number', 'Source map must have a version number');

// 6. Verify readInitialCoverage extracts initial coverage metadata
const initialCoverage = readInitialCoverage(instrumented);
assert.ok(initialCoverage, 'readInitialCoverage must extract coverage object from instrumented code');
assert.equal(initialCoverage.path, filename, 'Extracted initial coverage path must match');

// 7. Verify instrumentCode helper with ES modules and options
const esmSnippet = `
export function multiply(x, y) {
  return x * y;
}
`;
const result = instrumentCode(esmSnippet, 'calc.js', { esModules: true, produceSourceMap: true });
assert.ok(result.code.includes('multiply'), 'Instrumented ESM code must contain function name');
assert.equal(result.fileCoverage.path, 'calc.js', 'File coverage path must match calc.js');
assert.ok(result.sourceMap, 'Source map must be generated');

// 8. Verify instrumentSync throws error on syntax error
const invalidCode = `function invalid( { return ;`;
assert.throws(
  () => {
    instrumenter.instrumentSync(invalidCode, 'invalid.js');
  },
  (err) => {
    return err instanceof Error;
  },
  'instrumentSync must throw an error when given syntactically invalid code'
);

// 9. Verify instrumented code execution in VM records coverage hits
const execSnippet = `
function compute(n) {
  if (n > 5) {
    return n * 2;
  }
  return n;
}
compute(10);
`;
const inst2 = createInstrumenter({ coverageVariable: '__custom_cov__' });
const instCode2 = inst2.instrumentSync(execSnippet, 'compute.js');
const sandbox = { __custom_cov__: {} };
vm.createContext(sandbox);
vm.runInContext(instCode2, sandbox);

const recordedCoverage = sandbox.__custom_cov__['compute.js'];
assert.ok(recordedCoverage, 'Sandbox coverage variable must record coverage for compute.js');
assert.ok(recordedCoverage.s['0'] >= 1 || recordedCoverage.s['1'] >= 1, 'Statement hit counters must be incremented upon execution');

console.log('CONTRACT PASS: istanbul-lib-instrument createInstrumenter, instrumentSync, and lastFileCoverage verified');