Error in Closure Compiler when compiling in advanced mode

Question

When I use Closure Compiler I get the following errors when compiling in advanced mode: (in simple- and Whitespace only mode the code gets not problems at all)

JSC_REDECLARED_VARIABLE: Redeclared variable: e at line 31 character 9
} catch (e) {
         ^
JSC_REDECLARED_VARIABLE: Redeclared variable: e at line 34 character 9
} catch (e) {
         ^

The code described by Closure Compiler as an error is the following:

function getXMLHttp() {
    var xmlHttp;
    try {
        xmlHttp = new XMLHttpRequest();
    } catch (e) {
        try {
            xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (e) {
            try {
                xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
            } catch (e) {
                return false;
            }
        }
    }
    return xmlHttp;
}

Why this problem occurs when I use Advanced mode, I don't know. I have no idea why the compiler says this is an error as well.

(Quick note: This code is just a simple XMLrequest for an PHP file in a error catch handling for handling IE. All my JavaScript code works as it should be.)

Use the variables `e`, `f` and `g` the `catch` blocks. See if that works. — Blender, Jul 15 '12 at 20:22
Why do you need to compile this in advanced mode? It results in no code. — Esailija, Jul 15 '12 at 20:31
@user1426486 `Your code compiled to down to 0 bytes. Perhaps you should export some functions? Learn more` I guess 0 bytes is smaller then. — Esailija, Jul 15 '12 at 21:42
AFAIK, only Advanced Mode does variable re-declaration checking, so you don't get this warning in Simple Mode. — Stephen Chung, Jul 17 '12 at 03:13
@StephenChung But then this happens: Your code compiled to down to 0 bytes. Perhaps you should export some functions? — Jason Stackhouse, Jul 19 '12 at 11:06
Of course. You have one function definition, and the function is never used. It is eliminated as dead code in Advanced Mode. This has nothing to do with the variable re-declaration warning. — Stephen Chung, Jul 19 '12 at 14:24
@StephenChung All my functions in my JavaScript is used. What do you mean? — Jason Stackhouse, Jul 19 '12 at 18:43
If your functions are used, then what do you mean by it compiling down to 0 bytes? — Stephen Chung, Jul 20 '12 at 02:04
@StephenChung Well, I don't use my functions inside my JavaScript, but in the HTML. I don't see the problem. I tried running it in JSLint and stuff but I cannot see any problems. — Jason Stackhouse, Jul 20 '12 at 13:21
The Closure Compiler does not see your HTML. Therefore, to the compiler, your functions are never used. So, dead-code elimination kicks in and removes all your code. This is the correct behavior. You need to "use" your functions inside your JavaScript in order to avoid them being eliminated as dead code. One way is to "export" your functions -- check the on-line docs to learn how. — Stephen Chung, Jul 21 '12 at 07:53
I think your problem has nothing to do with your question -- which is a warning on re-declared variables. — Stephen Chung, Jul 21 '12 at 07:55

score 2 · Accepted Answer · answered Jul 15 '12 at 20:22

2

You can easily get rid of it by using e, e2 and e3 for your exceptions. That's dirty but still a good workaround.

answered Jul 15 '12 at 20:22

ThiefMaster

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score 0 · Answer 2 · answered Jul 15 '12 at 20:30

The way you use e, there will be no problem and your code should work as expected, however, if you would want to do something like this:

try {
    xmlHttp = new XMLHttpRequest();
} catch (e) {
    try {
        xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (e) {
        try {
            xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
        } catch (e) {
            return false;
        }
    }
    console.log(e); // Now there is a chance that e refer to the "wrong" exception
}

The closure compiler anticipates this, and throws a warning about it, since you re-declare e for every catch-statement.

As ThiefMaster suggest, you could simply rename the variables to e2, e3 and so forth if you want to get rid of the warning.

Error in Closure Compiler when compiling in advanced mode

2 Answers2