Unlike PHP, JavaScript generally does not require semicolons if you have only one statement on a line. Therefore, the following is valid:
x += 10
However, when you wish to place more than one statement on a line, they must be separated with semicolons, like this:
x += 10; y -= 5; z = 0
You can normally leave off the final semicolon, because the newline terminates the final statement.
There are exceptions to the semicolon rule. If you write JavaScript bookmarklets, or you end a statement with a variable or function reference and the first character of the next line is a left parenthesis or bracket, you must remember to append a semicolon or the JavaScript will fail. So, if in doubt, use a semicolon.