Q615 How do I know when to include the semi-colon (;) at the end of a line?

Semi-colons are optional. However, if you join two statements onto one line then you'll need a spearating semi-colon:

var x = 1; x = x + 1;

Which is the same as:

var x = 1
x = x + 1

It is always best to include semicolons:

var x = 1;
x = x + 1;