First prompt example

Technically he should use a semi-colon after the function call, but since it was the ONLY function being called on the page it wasn't exactly necessary. Semi-colons are statement separators. That means if he had ANOTHER statement following the one he left the semi-colon off, he would have most likely ran into a syntax, or other, error.

Here's a pretty decent example

prompt('hello')  alert('yo');

If you run this code you'll encounter a Syntax error because there is no semi-colon after the prompt function. i.e. there is nothing to separate these two function calls.

However, if you run this code:

prompt('hello');alert('yo');

With no spaces between the two functions you'll still achieve the desired results of a prompt, followed by the alert.

Best rule of thumb is to just use semi-colons, period.