Passing arrow functions (aka “lambdas”) to Mocha is discouraged. Lambdas lexically bind this and cannot access the Mocha context. For example, the following code will fail:

describe('my suite', () => {
  it('my test', () => {
    // should set the timeout of this test to 1000 ms; instead will fail

If you do not need to use Mochas context, lambdas should work. Be aware that using lambdas will be more painful to refactor if the need eventually arises!

Source: Mocha - the fun, simple, flexible JavaScript test framework

I knew this reason, but didn’t have a citation. Finally found it.