SeleniumHQ/selenium · error · TypeError

Level must be >= 0

Error message

Level must be >= 0

What it means

Thrown by the Level constructor (logging.js) when a custom log level is created with a negative numeric value. Log levels are ordered by non-negative numeric severity (OFF=Infinity down to ALL=0), so negatives are meaningless. It is a TypeError, not a WebDriver protocol error.

Source

Thrown at javascript/selenium-webdriver/lib/logging.js:79

 * APIs exposed by this module for it are non-frozen. This module will be
 * updated, possibly breaking backwards-compatibility, once logging is
 * officially defined by the
 * [W3C WebDriver spec](http://www.w3.org/TR/webdriver/).
 */

/**
 * Defines a message level that may be used to control logging output.
 *
 * @final
 */
class Level {
  /**
   * @param {string} name the level's name.
   * @param {number} level the level's numeric value.
   */
  constructor(name, level) {
    if (level < 0) {
      throw new TypeError('Level must be >= 0')
    }

    /** @private {string} */
    this.name_ = name

    /** @private {number} */
    this.value_ = level
  }

  /** This logger's name. */
  get name() {
    return this.name_
  }

  /** The numeric log level. */
  get value() {
    return this.value_
  }

View on GitHub (pinned to aa36b38e69)

Solutions

1. Use a non-negative value: new logging.Level('CUSTOM', 500).
2. Clamp derived values: Math.max(0, base - offset).
3. Reuse the built-in constants (Level.DEBUG/INFO/WARNING/SEVERE/OFF/ALL) instead of constructing custom levels.
4. Validate config input is a finite non-negative number before constructing the Level.

Example fix

// before
const lvl = new logging.Level('trace', base.value - 1000); // can be negative

// after
const lvl = new logging.Level('trace', Math.max(0, base.value - 1000));

function makeLevel(name, value) {
  if (!Number.isFinite(value) || value < 0) {
    throw new TypeError(`level value must be a finite non-negative number, got ${value}`);
  }
  return new logging.Level(name, value);
}

function isValidLevelValue(v) {
  return typeof v === 'number' && Number.isFinite(v) && v >= 0;
}

try {
  return new logging.Level(name, derived);
} catch (e) {
  if (e instanceof TypeError && /Level must be >= 0/.test(e.message)) {
    return new logging.Level(name, Math.max(0, derived));
  }
  throw e;
}

When it happens

Trigger: new logging.Level('CUSTOM', -1); computing a level from user input or a delta without clamping, e.g. new Level(name, base.value - offset) where offset exceeds base.

Common situations: Subtracting severities to derive a 'quieter' level and going below zero; reading a level number from env/config that defaults to -1 as a sentinel; porting code from a library whose levels start at 1.

Related errors

• not a string or number
• log_level must be one of: {', '.join(levels)}
• Invalid URL: ${aUrl}
• no capabilities provided for merge
• Capability keys must be strings: ${typeof key}

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/9974a067d1e786b9. Report an issue: GitHub.