javascript - Try Catch not catching syntax error in Chrome - Stack Overflow

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I'm trying to troubleshoot a problem with someone else's JavaScript file, and they declared a function like the following.

function window.confirm(str) {
..... code here .....
}

This works fine with IE, but in Google Chrome, it throws an uncaught syntax error on the period in window.confirm. I tried to put a try catch around it like below, but that didn't work, same syntax error. It then won't let me use any functions defined in that JavaScript file.

try {
    var window.confirm = function(str) {
        ..... code here .....
    };
}
catch(e) {}

I also tried to change the declaration to a variable, like below, but that didn't work either. Same error.

var window.confirm = function(str) {
    ..... code here .....
};

Is there a way to catch this in Chrome?

I'm trying to troubleshoot a problem with someone else's JavaScript file, and they declared a function like the following.

function window.confirm(str) {
..... code here .....
}

This works fine with IE, but in Google Chrome, it throws an uncaught syntax error on the period in window.confirm. I tried to put a try catch around it like below, but that didn't work, same syntax error. It then won't let me use any functions defined in that JavaScript file.

try {
    var window.confirm = function(str) {
        ..... code here .....
    };
}
catch(e) {}

I also tried to change the declaration to a variable, like below, but that didn't work either. Same error.

var window.confirm = function(str) {
    ..... code here .....
};

Is there a way to catch this in Chrome?

• javascript
• function
• google-chrome

Share Improve this question Follow edited Nov 7, 2012 at 16:11 Ry-♦ 225k5656 gold badges492492 silver badges498498 bronze badges asked Nov 7, 2012 at 16:06 AndrewAndrew 47055 silver badges1818 bronze badges 6

• 1 Er... you can't catch syntax errors. How about fixing it instead? window.confirm = function(str) {...}; – Ry- ♦ Commented Nov 7, 2012 at 16:08
• People aren't going to like me for pointing out that you can catch syntax errors if you parse the code with eval(). That would be a terrible idea in this case however because that is a syntax error and it should just be fixed. – Pointy Commented Nov 7, 2012 at 16:09
• 2 @Pointy: Eh, I'd count that as "catching a SyntaxError" as opposed to "catching a syntax error". – Ry- ♦ Commented Nov 7, 2012 at 16:10
• @minitech Ha ha good point :-) It's not something I'd normally bring up but I've been working with a JavaScript templating system lately and now I have to deal with caught "SyntaxErrors" all the time. – Pointy Commented Nov 7, 2012 at 16:18
• @minitech - Thanks for the edit. I know it was a syntax error, but I didn't know how to fix it. It worked fine in IE which is why I was just trying to catch it for the other browsers so nothing broke. everyoneelse - Thank for for the knowledge that try/catch only catches runtime, I wasn't aware of that. – Andrew Commented Nov 12, 2012 at 16:50

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4 Answers 4

Sorted by: Reset to default 3

function window.confirm(str) and var window.confirm ... are invalid. instead, try:

window.confirm = function(str){
..... code here .....
}

Two points :

• try/catch are used to detect execution errors, not pilation ones. Don't deploy code with syntax errors.

• function window.confirm(){ is a MS idiom you must avoid. You may use window.confirm = function() { or, if you're in the global scope, simply var confirm = function() {.

If you absolutely need to try and catch "Uncaught SyntaxError" errors, then you'll have to rely on JavaScript's hidden-under-the-floorboard misunderstood stepchild eval() function.

In the sample code below, obj will only be updated if the ining string evaluates to a valid object; otherwise obj will default to {}. Let's assume $someTextField pulls in settings entered in a CMS textarea field, and the content editor has entered this malformed string in the field:

{
  "foo": "bar
}

Sample code:

var oSettings = {};

// Returns "Uncaught SyntaxError: Unexpected token ILLEGAL(…)"
try {
  oSettings = <?php echo $someTextField ?>;
} catch(e) {}

// This works
try {
  eval('oSettings = <?php echo $someTextField ?>');
} catch(e) {}

Keep in mind that if the string to be evaluated is ing from some external source (e.g., CMS) and you have no control over it as in this example, then you'll need to do three things:

1. Detect and prevent execution of JavaScript code that could potentially wreck hell on your app.

2. Escape quotes in the ining string to avoid conflicts with the JavaScript quotes. Since we're using PHP in this example, it would be better to do something like this:

   eval('oSettings = <?php echo addslashes($someTextField) ?>');

3. Remove line breaks since the evaluated string needs to be on one single line. It would be even better to do something like this:

   eval('oSettings = <?php echo addslashes(str_replace(array("\r\n", "\n", "\r"), "", $someTextField)) ?>');

You must fix the syntax errors. Try/catch will not catch syntax errors because syntax errors are pile time errors and try/catch can only catch runtime errors.

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