mochajs/mocha · error · TypeError

Missing runner argument

Error message

Missing runner argument

What it means

The `Base` reporter constructor requires a runner instance as its first argument because it immediately reads `runner.stats`, attaches event listeners, and tracks failures. A `TypeError` is thrown when the reporter is instantiated without one, which almost always means Mocha internals were wired incorrectly.

Source

Thrown at lib/reporters/base.js:63

/**
 * @abstract
 * @description
 * All other reporters generally inherit from this reporter.
 */
export class Base {
  /**
   * Constructs a new `Base` reporter instance.
   *
   * @public
   * @memberof Mocha.reporters
   * @param {Runner} runner - Instance triggers reporter actions.
   * @param {Object} [options] - runner options
   */
  constructor(runner, options) {
    var failures = (this.failures = []);

    if (!runner) {
      throw new TypeError("Missing runner argument");
    }
    this.options = options || {};
    this.runner = runner;
    this.stats = runner.stats; // assigned so Reporters keep a closer reference

    var maxDiffSizeOpt =
      this.options.reporterOption && this.options.reporterOption.maxDiffSize;
    if (maxDiffSizeOpt !== undefined && !isNaN(Number(maxDiffSizeOpt))) {
      Base.maxDiffSize = Number(maxDiffSizeOpt);
    }

    runner.on(EVENT_TEST_PASS, function (test) {
      if (test.duration > test.slow()) {
        test.speed = "slow";
      } else if (test.duration > test.slow() / 2) {
        test.speed = "medium";
      } else {
        test.speed = "fast";

View on GitHub (pinned to 6bcbee4fd9)

Solutions

1. Always pass the runner: `new MyReporter(runner, options)`.
2. In custom reporter subclasses, forward arguments to the base class: `constructor(runner, options) { super(runner, options); ... }`.
3. Let Mocha instantiate reporters via `new Mocha({reporter: 'spec'}).run()` instead of constructing them yourself.

Example fix

// before
constructor(options) {
  super(); // TypeError: Missing runner argument
}

// after
constructor(runner, options) {
  super(runner, options);
}

if (typeof runner === 'undefined' || runner === null) {
  throw new TypeError('Cannot create reporter: runner is required');
}

const isRunner = (r) => r != null && typeof r.on === 'function' && typeof r.emit === 'function';

try {
  const reporter = new Base(runner, options);
} catch (err) {
  if (err instanceof TypeError && err.message === 'Missing runner argument') {
    console.error('Reporter requires a runner from mocha.run()');
  }
  throw err;
}

When it happens

Trigger: `new (require('mocha/lib/reporters/base'))()` with no runner; instantiating a custom reporter directly without passing the runner; a wrapper/adapter class forgetting to forward the runner to `super()`.

Common situations: Custom reporters calling `super()` without forwarding `runner`; test tooling that constructs reporters manually outside of `mocha.run()`; framework integrations creating a reporter before the runner exists.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Not enough non-option arguments: got 0, need at least 1
• Not enough arguments following: ${name}
• invalid or unsupported TAP version: ${JSON.stringify(tapVers
• Missing runner argument
• Invalid argument; expected a non-empty object

AI-assisted analysis of mochajs/mocha@6bcbee4fd9 (2026-09-01). Data as JSON: /api/errors/dafdce161a2c82f6. Report an issue: GitHub.