JS Invalid left-hand side expression in postfix operation?

Question

I am playing with a javascript and am running into an error. The error is this:

Invalid left-hand side expression in postfix operation.

And the script is long but I think this is this issue. The weird thing is this works when I run it locally, but when it is packaged, using asset_packager, it fails.

Any ideas why I might be getting this error?

UPDATE: After doing more research I found this function. The error seems to happen after in the "while" statement and I assume it's the "++ + a + ". This is a plugin so I didn't want to go messing with the code...but do you thing this could be it?

m.getInternetExplorerMajorVersion = function() {
            var a = m.getInternetExplorerMajorVersion.cached = typeof m.getInternetExplorerMajorVersion.cached != "undefined" ? m.getInternetExplorerMajorVersion.cached : function() {
                var a = 3, b = d.createElement("div"), c = b.getElementsByTagName("i");
                while ((b.innerHTML = "<!--[if gt IE "++ + a + "]><i></i><![endif]-->") && c[0])
Uncaught ReferenceError: Invalid left-hand side expression in postfix operation
                    ;
                return a > 4 ? a : !1
            }();
            return a
        }

Answer 1

You didn't say which plugin was that but I was dealing with the same problem, Jeffrey, with I think the same plugin, because my code was looking almost the same.

I followed your lead. The plugin was History.js, from page: https://github.com/browserstate/History.js/ and I was using bundled html4+html5 version, which was minimized.

I changed that fragment

innerHTML="<!--[if gt IE "+ ++a+"]><i></i><![endif]-->"

into

innerHTML="<!--[if gt IE "+ (++a)+"]><i></i><![endif]-->"

And it did the job!

I started to wonder what exactly problem lied in. The most important suspect was of course minification "compression". In normal situation following code is correct

var a=0; "begining of string "+ ++a+" the rest of string";

And returns "begining of string 1 the rest of string"

However the minification gets rid of white space and turns it into something that is understood by a browser as a:

var a=0;"begining of string "+++a+" the rest of string";

What gives us error Uncaught ReferenceError: Invalid left-hand side expression in postfix operation

EDIT: As Sam pointed out, the issue was caused by minifcation, and not than gzip compression, of course :)