refactor: fix code style and typos

[fmalcher]

• use let and const and avoid evil var
• fix typos, add full stop after sentences
• consistently use single quotes
• use triple equals

Open questions

I'd recommend to refactor the following, but I did not know whether you had a good reason for them.

Checks for undefined

You often do checks for undefined values like this:

if (listeners[key] === undefined) {
  return;
}

Is there a specific reason why you soecifically check for undefined and not for truthyness/falsyness?

if (!listeners[key]) {
  return;
}

const self = this

Each function has a self variable to save the this context of the function – fair enough! However, this would not be necessary if you used arrow functions all the way for the inner functions. Is there a reason why you chose to do so?

[krocheck]

@haakonnessjoen do we want to move forward with this?