GoogleChrome/lighthouse · error · Error

Invalid output mode: ${outputMode}

Error message

Invalid output mode: ${outputMode}

What it means

Thrown by ReportGenerator.generateReport when an output mode string is not one of the three supported values: 'html', 'csv', or 'json'. The mode is matched by strict equality, so it is case-sensitive and typo-sensitive. The offending value is concatenated into the message for diagnostics.

Source

Thrown at report/generator/report-generator.js:187

      if (outputMode === 'html') {
        if (ReportGenerator.isFlowResult(result)) {
          return ReportGenerator.generateFlowReportHtml(result);
        }
        return ReportGenerator.generateReportHtml(result);
      }
      // CSV report.
      if (outputMode === 'csv') {
        if (ReportGenerator.isFlowResult(result)) {
          throw new Error('CSV output is not support for user flows');
        }
        return ReportGenerator.generateReportCSV(result);
      }
      // JSON report.
      if (outputMode === 'json') {
        return JSON.stringify(result, null, 2);
      }

      throw new Error('Invalid output mode: ' + outputMode);
    });

    return outputAsArray ? output : output[0];
  }
}

export {ReportGenerator};

View on GitHub (pinned to 9515cd4e58)

Solutions

1. Use exactly one of the lowercase literals: 'html', 'csv', or 'json'.
2. Validate/whitelist the mode against ['html','csv','json'] before calling generateReport and fall back to a default like 'html'.
3. Normalize input with mode.toLowerCase().trim() if the value originates from user input.

Example fix

// before
ReportGenerator.generateReport(result, mode); // mode could be 'HTML'

// after
const VALID = ['html', 'csv', 'json'];
const mode = VALID.includes(String(rawMode).toLowerCase()) ? rawMode.toLowerCase() : 'html';
ReportGenerator.generateReport(result, mode);

const VALID_MODES = ['html', 'csv', 'json'];
function normalizeMode(raw) {
  const m = String(raw).toLowerCase().trim();
  return VALID_MODES.includes(m) ? m : 'html';
}
ReportGenerator.generateReport(result, normalizeMode(userMode));

/** @param {unknown} m */
function isValidMode(m) {
  return typeof m === 'string' && ['html', 'csv', 'json'].includes(m);
}

try {
  ReportGenerator.generateReport(result, mode);
} catch (err) {
  if (err.message.startsWith('Invalid output mode:')) {
    ReportGenerator.generateReport(result, 'html'); // safe default
  } else throw err;
}

When it happens

Trigger: Calling ReportGenerator.generateReport(result, outputMode) with outputMode equal to e.g. 'xml', 'HTML', 'md', or an empty/undefined value. Also triggered when passing a non-string scalar that is then compared.

Common situations: Typos or casing mistakes ('HTML' instead of 'html'); passing a value read from an unvalidated CLI flag, env var, or config file; defaulting a missing mode to undefined. Using a newer/imagined format name not implemented by this generator.

Related errors

• CSV output is not support for user flows
• Please provide a url
• Invalid value: Argument must be a string or a boolean
• Invalid values. Argument 'output' must be an array from choi
• "${str}" is not a valid 'output' value. Argument 'output' mu

AI-assisted analysis of GoogleChrome/lighthouse@9515cd4e58 (2026-08-13). Data as JSON: /api/errors/51d40aef74ea0301. Report an issue: GitHub.