Javascript packing problem

Question

I have a minified/packed javascript file which is causing problems. The issue is that the non-packed input file has some missing semicolons somewhere which aren't a problem when there's line breaks, but when the file is packed, the line breaks are removed and that causes a parser error. For example:

//input
var x = function() {
    doSomething();
}  // note: no semicolon
var y = 'y';

//----
// output
var x=function(){doSomething();}var y='y';
//                  error here: ^

The strange thing is that when I do a replace on the output file to replace all semicolons with a semicolon and a new line, the file works! This is making it ludicrously hard to find the error, since AFAIK, I can't think of any situation where a line break after a semicolon should change anything. Any ideas about why doing this replace would make it work?

Answer 1

Uh... Have you tried JSLint?

Answer 2

When there's a line break, there's an implied semi-colon.

Answer 3

Use jslint to check your code. If you do this and get it passing with regards to semicolons, it should pack correctly.

In JavaScript, semicolons are implicitly added at newlines. This introduces situations that can be ambiguous. This blog post: http://robertnyman.com/2008/10/16/beware-of-javascript-semicolon-insertion/ describes the problem succinctly and gives an example.

Answer 4

JSlint is a good solution. Also, some code editors will find these kinds of errors for you. If I recall correctly, NetBeans catches these in realtime, as you type. I believe Komodo and Aptana do as well.