Detecting Multiple Calls to done()

If you use callback-based async tests, Mocha will throw an error if done() is called multiple times. This is handy for catching accidental double callbacks.

it("double done", function (done) {
  // Calling `done()` twice is an error
  setImmediate(done);
  setImmediate(done);
});

Running the above test will give you the below error message:

$ ./node_modules/.bin/mocha mocha.test.js


  ✓ double done
  1) double done

  1 passing (6ms)
  1 failing

  1) double done:
     Error: done() called multiple times
      at Object.<anonymous> (mocha.test.js:1:63)
      at require (internal/module.js:11:18)
      at Array.forEach (<anonymous>)
      at startup (bootstrap_node.js:187:16)
      at bootstrap_node.js:608:3