example-test.js

const assert = require('assert');
const sandbox = require('./sandbox');

function myAsyncFn (callback) {
  setTimeout(() => {
    callback(null, 5);
  }, 500);
}

describe('async things', () => {
  it('does not report nicely', (done) => {
    myAsyncFn((err, value) => {
      assert.equal(value, 4); // blows up with an unhandled exception crashing mocha process
      done();
    });
  });
  it('uses sandbox, so reports nicely', (done) => {
    myAsyncFn(sandbox(done, (err, value) => {
      assert.equal(value, 4); // fails with a "nice" mocha failure report
      done();
    }));
  });
});

sandbox.js

/**
 * Helper function to allow mocha to nicely report errors in assertions
 * Normally an error thrown in an async callback will result in an
 * unhandledException. By wrapping in a try/catch block and passing any
 * assertion errors back to the mocha callback we can utilise mocha's
 * error reporting properly.
 */
module.exports = function (done, callback) {
  return function () {
    try {
      callback.apply(null, arguments);
    } catch (e) {
      done(e);
    }
  };
};