TypeError when calling this.timeout() inside an arrow function test in Mocha

Why the error occurs

In Mocha, this.timeout() relies on the test‑suite context object that is bound to the function executing the test or hook. Arrow functions do not create their own this binding; they inherit this from the surrounding scope, which is usually the module exports object. Consequently, calling this.timeout(5000) inside an arrow‑function test throws a TypeError: this.timeout is not a function.

Confirmed facts

• Mocha accepts a timeout value as the third argument to it() (or describe()) and treats it as the per‑test (or per‑suite) limit, independent of this.
• When this.timeout() is called inside a regular‑function describe callback, the value becomes the default timeout for all tests and hooks (beforeEach, afterEach, before, after) defined within that suite, unless they explicitly override it.
• The timeout property in a Mocha configuration file (.mocharc.js, mocharc.json, or mocha.opts) is functionally identical to the CLI flag --timeout; it sets the global default that can be overridden at the suite or test level.

Steps to set a per‑test timeout inside an arrow‑function test

1. Pass the desired timeout (in milliseconds) as the third argument to it:

it('should complete within 5 seconds', () => {
    // async work that may take up to ~5 s
    return new Promise(resolve => setTimeout(resolve, 4500));
  }, 5000);

2. If you need a suite‑wide default that applies to all arrow‑function tests, set it once in a regular‑function describe block:

describe('suite with arrow tests', function () {
    // regular function so `this` is the Mocha context
    this.timeout(3000); // default for this suite

    it('quick test', () => { return Promise.resolve(); });
    it('longer test', () => { return new Promise(r => setTimeout(r, 2500)); }); // uses suite default
  });

3. To avoid editing every test file, you can also define the default in a config file:

// .mocharc.json
{
  "timeout": 3000
}

4. Run Mocha as usual; the config‑file value behaves exactly like invoking mocha --timeout 3000.

Verification checklist

• Confirm that a test using the third‑argument timeout passes when its async work finishes within the specified limit and fails with a timeout error when it exceeds it.
• Confirm that removing the third‑argument timeout causes the test to fall back to the suite default (set via this.timeout() in a regular‑function describe or via config).
• Confirm that a hook (beforeEach, etc.) does **not** inherit a test‑level timeout set via the third argument; it only receives the suite default unless you set it explicitly on the hook.