jestjs/jest · error · MalformedPackageJsonError

There is malformed json in

Error message

There is malformed json in ${packageJsonPath}

What it means

create-jest reads and JSON.parses package.json before generating the Jest config; if parsing throws (syntax error, trailing comma, unquoted key, BOM, or empty file), it wraps the failure in MalformedPackageJsonError naming the offending path. The init cannot proceed because it needs to inspect `scripts.test` and the `jest`/`type` fields and may rewrite the file.

Solutions

1. Run `node -e "JSON.parse(require('fs').readFileSync('package.json','utf8'))"` or `npx jsonlint package.json` to locate the exact syntax error and fix it.
2. Re-run `jest --init` after the JSON parses cleanly.
3. If the file was truncated, restore it from version control (`git checkout -- package.json`) and re-apply intended edits carefully.

Example fix

// before (package.json)
{
  "name": "app",
  "scripts": { "test": "jest", },   // trailing comma
}

// after
{
  "name": "app",
  "scripts": { "test": "jest" }
}

// Validate package.json parses before invoking jest --init.
import * as fs from 'node:fs';

function validatePackageJson(filePath) {
  try {
    JSON.parse(fs.readFileSync(filePath, 'utf8'));
  } catch (e) {
    throw new Error(`${filePath} is not valid JSON: ${e.message}`);
  }
}

// Or lint from the CLI: npx jsonlint package.json

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

When it happens

Trigger: `jest --init` while package.json contains invalid JSON: trailing commas, single-quoted strings, comments, a stray character, or a zero-byte file. The try/catch around JSON.parse(readFileSync(...)) catches any throw and rethrows this error.

Common situations: Hand-edited package.json with a JSON5 feature (comments/trailing commas) not valid in strict JSON; a botched merge/rebase leaving a conflict marker or duplicate key; a file accidentally overwritten or truncated; copy-pasting config snippets with unquoted keys.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• Could not find a "package.json" file in
• Cannot parse as JSON
• ● Validation Error: Configuration in

AI-assisted analysis of jestjs/jest@8e6d128e4a (2026-08-10). Data as JSON: /api/errors/cbc3984422c1e85f. Report an issue: GitHub.

Appendix: 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',
    });

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