alert('Hi!') is a function call that alerts 'Hi' and returns nothing (undefined).
onclick expects to get a function and you are passing the function call's result which is undefined.
Since JavaScript is not a strong typed framework you don't get an error on bad assignments.
So why does the following work:
It's because the html parser (in this case, your browser) does some work behind the scenes and wraps the call with a function.
