What causes "Failed to parse ${path.basename(configFile)}" in parcel-bundler/parcel?

Open the file named in the message and fix the syntax at the reported line/column.

parcel-bundler/parcel · error · ThrowableDiagnostic

Failed to parse ${path.basename(configFile)}

Error message

Failed to parse ${path.basename(configFile)}

What it means

Thrown by @parcel/utils readConfig when a JSON/JSON5 config file fails to parse. Only raised for files with a .json extension or no extension (JSON5-style files); other extensions rethrow the original parser error unwrapped. The ThrowableDiagnostic includes a code frame pinpointing e.lineNumber/e.columnNumber.

Source

Thrown at packages/core/utils/src/config.js:134

    let config;
    if (parse === false) {
      config = configContent;
    } else {
      let extname = path.extname(configFile).slice(1);
      let parse = opts?.parser ?? getParser(extname);
      try {
        config = parse(configContent);
      } catch (e) {
        if (extname !== '' && extname !== 'json') {
          throw e;
        }

        let pos = {
          line: e.lineNumber,
          column: e.columnNumber,
        };

        throw new ThrowableDiagnostic({
          diagnostic: {
            message: `Failed to parse ${path.basename(configFile)}`,
            origin: '@parcel/utils',
            codeFrames: [
              {
                language: 'json5',
                filePath: configFile,
                code: configContent,
                codeHighlights: [
                  {
                    start: pos,
                    end: pos,
                    message: e.message,
                  },
                ],
              },
            ],
          },

View on GitHub (pinned to 59484858a1)

Solutions

Open the file named in the message and fix the syntax at the reported line/column.
Validate the file with `node -e "JSON.parse(require('fs').readFileSync('FILE','utf8'))"` (for strict JSON).
Rename to .json5 / .jsonc if you intentionally use comments/trailing commas and ensure the right parser is registered.
Remove git merge conflict markers or BOM before re-running.

Example fix

// before — .eslintrc.json (invalid: trailing comma)
{
  "extends": ["parcel"],
}

// after
{
  "extends": ["parcel"]
}

Defensive patterns

Strategy: validation

Validate before calling

// Validate a JSON config before passing it to Parcel.
const fs = require('fs');
function validateJsonConfig(file) {
  const txt = fs.readFileSync(file, 'utf8');
  try { JSON.parse(txt); }
  catch (e) { throw new Error(`${file} is invalid JSON: ${e.message}`); }
}

Try / catch

try {
  const cfg = await readConfig(fs, configFile);
} catch (e) {
  if (e?.diagnostic?.message?.startsWith('Failed to parse')) {
    // surface the code frame, prompt user to fix the file
  } else throw e;
}

Prevention

Lint JSON configs in CI with a JSON parser before build.
Use .jsonc/.json5 explicitly when you need comments/trailing commas and register the matching parser.
Add editor schema validation for .parcelrc/tsconfig etc.

When it happens

Trigger: Parcel loads a .parcelrc, .eslintrc, tsconfig.json, or other JSON config that contains a syntax error (trailing comma, unquoted key, single quotes, unclosed brace) and the registered parser throws.

Common situations: Hand-editing JSON and introducing a trailing comma or comment; copying JSON5 syntax into a strict .json file; BOM/encoding issues; git merge conflicts leaving conflict markers in the config.

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

AI-assisted analysis of parcel-bundler/parcel@59484858a1 (2026-08-13). Data as JSON: /api/errors/e669aa309c9aaebf. Report an issue: GitHub.