jestjs/jest · error · MalformedPackageJsonError

There is malformed json in ${packageJsonPath}

Error message

There is malformed json in ${packageJsonPath}

What it means

After confirming package.json exists, runCreate reads and JSON.parses it; any parse failure is caught and rethrown as MalformedPackageJsonError. create-jest needs a real JS object to read/write scripts and the `jest`/`type` fields, so it refuses to proceed on invalid JSON rather than silently corrupting the file on write.

Source

Thrown at packages/create-jest/src/runCreate.ts:68

export async function runCreate(rootDir = process.cwd()): Promise<void> {
  rootDir = tryRealpath(rootDir);
  // prerequisite checks
  const projectPackageJsonPath = path.join(rootDir, PACKAGE_JSON);

  if (!fs.existsSync(projectPackageJsonPath)) {
    throw new NotFoundPackageJsonError(rootDir);
  }

  const questions = [...defaultQuestions];
  let hasJestProperty = false;
  let projectPackageJson: ProjectPackageJson;

  try {
    projectPackageJson = JSON.parse(
      fs.readFileSync(projectPackageJsonPath, 'utf8'),
    ) as ProjectPackageJson;
  } catch {
    throw new MalformedPackageJsonError(projectPackageJsonPath);
  }

  if (projectPackageJson.jest) {
    hasJestProperty = true;
  }

  const existingJestConfigExt = JEST_CONFIG_EXT_ORDER.find(ext =>
    fs.existsSync(path.join(rootDir, getConfigFilename(ext))),
  );

  if (hasJestProperty || existingJestConfigExt != null) {
    const result: {continue: boolean} = await prompts({
      initial: true,
      message:
        'It seems that you already have a jest configuration, do you want to override it?',
      name: 'continue',
      type: 'confirm',
    });

View on GitHub (pinned to f49721c78e)

Solutions

1. Run a JSON validator on the file: `node -e "JSON.parse(require('fs').readFileSync('package.json','utf8'))"` to surface the exact parse error with a line/column.
2. Open package.json and fix the flagged syntax (remove trailing commas, quote keys, remove comments).
3. Use an IDE JSON language server or `npx fixpack` to normalize the file, then re-run create-jest.
4. If a git merge left conflict markers (`<<<<<<<`), resolve the merge first.

Example fix

// before — package.json with trailing comma
{
  "name": "app",
  "version": "1.0.0",
}

// after
{
  "name": "app",
  "version": "1.0.0"
}

const fs = require('node:fs');
const text = fs.readFileSync('package.json', 'utf8');
try {
  JSON.parse(text);
} catch (e) {
  console.error('package.json is invalid JSON:', e.message);
  process.exit(1);
}
// safe to invoke create-jest

function isValidPackageJson(text: string): boolean {
  try { JSON.parse(text); return true; } catch { return false; }
}

try {
  await runCreate(rootDir);
} catch (e) {
  if (e instanceof MalformedPackageJsonError) {
    // point the user at the JSON parse error (re-parse to get line/col)
  }
  throw e;
}

When it happens

Trigger: package.json contains a JSON syntax error: trailing comma, single-quoted strings, unquoted keys, a // or /* */ comment, a stray BOM/character, or an unclosed brace. Any of these makes JSON.parse throw and runCreate rethrows MalformedPackageJsonError.

Common situations: Hand-editing package.json and leaving a trailing comma; JSON5-style config that Node tolerates via require() but raw JSON.parse does not; merge-conflict markers left in the file; IDE auto-formatting disabled.

Related errors

• Could not find a "package.json" file in ${rootDir}
• Cannot parse ${filePath} as JSON: ${error.message}
• babel-jest: Babel ignores ${slash(path.relative(cwd, filenam
• received value must be a mock or spy function
• received value must be a mock function

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/cbc3984422c1e85f.json. Report an issue: GitHub.