Why doesn't prompt() need a semi-colon to work?

Prompt() is function or method declaration which doesn't require semicolons. I think you may need one depending on how you use it. If you assign it to a variable you may have to end the line with a semicolon. Semi colons help end statements but methods aren't necessarily statements. It's more like an expression like an if statement.

However you can safely use them if you prefer. The Semi colons don't terminate lines of code, merely help separate them from the other! :-)

You don't need to add the semi-colon to the last statement in a script. But its just a good idea to do it so you don't forget elsewhere. As soon as he added the console.log after it then he had to add the semi-colon.

The semicolon is a way to separate statements. Technically, you don't need it on a single line program, or on the last line of a file (as there aren't any statements to separate at that point), but it's good practice to always use it at the end of a line. After all, you could add more code later and a missing semicolon there could result in a syntax error.

Technically you are not required to include semicolons as line ends anywhere in JavaScript-- it's a great practice though because many other programming languages due require semicolons