JavaScript ExceptionError Handling Not Working - Stack Overflow

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This might be a little hard to follow.

I've got a function inside an object:

f_openFRHandler:    function(input) {
            console.debug('f_openFRHandler');
            try{
                //throw 'foo';
                DragDrop.FileChanged(input);
                //foxyface.window.close();
            }
            catch(e){
                console.error(e);
                jQuery('#foxyface_open_errors').append('<div>Max local storage limit reached, unable to store new images in your browser. Please remove some images and try again.</div>');
            }
        },

inside the try block it calls:

this.FileChanged = function(input) {
            // FileUploadManager.addFileInput(input);
            console.debug(input);
            var files = input.files;
            for (var i = 0; i < files.length; i++) {
                var file = files[i];
                if (!file.type.match(/image.*/)) continue;

                var reader = new FileReader();
                reader.onload = (function(f, isLast) {
                    return function(e) {
                        if (files.length == 1) {
                            LocalStorageManager.addImage(f.name, e.target.result, false, true);
                            LocalStorageManager.loadCurrentImage();
                            //foxyface.window.close();
                        }
                        else {
                            FileUploadManager.addFileData(f, e.target.result); // add multiple files to list
                            if (isLast) setTimeout(function() { LocalStorageManager.loadCurrentImage() },100);
                        }
                    };
                })(file, i == files.length - 1);
                reader.readAsDataURL(file);
            }
            return true;

LocalStorageManager.addImage calls:

this.setItem = function(data){
                localStorage.setItem('ImageStore', $.json_encode(data));
        }

localStorage.setItem throws an error if too much local storage has been used. I want to catch that error in f_openFRHandler (first code sample), but it's being sent to the error console instead of the catch block. I tried the following code in my Firebug console to make sure I'm not crazy and it works as expected despite many levels of function nesting:

try{
    (function(){
        (function(){
            throw 'foo'
        })()
    })()
}
catch(e){
    console.debug(e)
}

Any ideas?

This might be a little hard to follow.

I've got a function inside an object:

f_openFRHandler:    function(input) {
            console.debug('f_openFRHandler');
            try{
                //throw 'foo';
                DragDrop.FileChanged(input);
                //foxyface.window.close();
            }
            catch(e){
                console.error(e);
                jQuery('#foxyface_open_errors').append('<div>Max local storage limit reached, unable to store new images in your browser. Please remove some images and try again.</div>');
            }
        },

inside the try block it calls:

this.FileChanged = function(input) {
            // FileUploadManager.addFileInput(input);
            console.debug(input);
            var files = input.files;
            for (var i = 0; i < files.length; i++) {
                var file = files[i];
                if (!file.type.match(/image.*/)) continue;

                var reader = new FileReader();
                reader.onload = (function(f, isLast) {
                    return function(e) {
                        if (files.length == 1) {
                            LocalStorageManager.addImage(f.name, e.target.result, false, true);
                            LocalStorageManager.loadCurrentImage();
                            //foxyface.window.close();
                        }
                        else {
                            FileUploadManager.addFileData(f, e.target.result); // add multiple files to list
                            if (isLast) setTimeout(function() { LocalStorageManager.loadCurrentImage() },100);
                        }
                    };
                })(file, i == files.length - 1);
                reader.readAsDataURL(file);
            }
            return true;

LocalStorageManager.addImage calls:

this.setItem = function(data){
                localStorage.setItem('ImageStore', $.json_encode(data));
        }

localStorage.setItem throws an error if too much local storage has been used. I want to catch that error in f_openFRHandler (first code sample), but it's being sent to the error console instead of the catch block. I tried the following code in my Firebug console to make sure I'm not crazy and it works as expected despite many levels of function nesting:

try{
    (function(){
        (function(){
            throw 'foo'
        })()
    })()
}
catch(e){
    console.debug(e)
}

Any ideas?

• javascript
• exception
• event-bubbling
• throw

Share Improve this question Follow edited Sep 4, 2012 at 22:17 animuson♦ 54.8k2828 gold badges142142 silver badges150150 bronze badges asked Jun 15, 2010 at 7:52 Seán HayesSeán Hayes 4,37055 gold badges3737 silver badges5353 bronze badges 3

• 3 This is a great example of how to ask a question about a plex situation providing the relevant information concisely. Nice one. – T.J. Crowder Commented Jun 15, 2010 at 8:03
• Ahh, I should've seen that. Thanks for the help guys. – Seán Hayes Commented Jun 16, 2010 at 2:38
• án: If one of the answers below helped you solve the problem, please accept one of them by clicking the check mark to the left of the top of the answer. That marks the question "answered". (stackoverflow./faq) – T.J. Crowder Commented Jun 16, 2010 at 9:14

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2 Answers 2

Sorted by: Reset to default 4

I think the problem is that the error is happening later, after your f_openFRHandler function has pleted. Note that the function where LocalStorageManager.addImage is being called is being set as the onload handler on the reader, not being called immediately. It gets called later, asynchronously, when the data is loaded.

You'll need to put your try..catch inside the anonymous function being created and assigned to that event, e.g.:

this.FileChanged = function(input) {
    // FileUploadManager.addFileInput(input);
    console.debug(input);
    var files = input.files;
    for (var i = 0; i < files.length; i++) {
        var file = files[i];
        if (!file.type.match(/image.*/)) continue;

        var reader = new FileReader();
        reader.onload = (function(f, isLast) {
            return function(e) {
                try {       // ADDED
                    if (files.length == 1) {
                        LocalStorageManager.addImage(f.name, e.target.result, false, true);
                        LocalStorageManager.loadCurrentImage();
                        //foxyface.window.close();
                    }
                    else {
                        FileUploadManager.addFileData(f, e.target.result); // add multiple files to list
                        if (isLast) setTimeout(function() { LocalStorageManager.loadCurrentImage() },100);
                    }
                }
                catch (err) { // ADDED
                    // YOUR HANDLING HERE
                }
            };
        })(file, i == files.length - 1);
        reader.readAsDataURL(file);
    }
    return true;
};

Your (excellent) test case makes the call synchronously, which is why the error is caught when you try it. This is a closer model to what's actually happening:

try{
    (function(){
        setTimeout(function(){  // Use setTimeout to model asynchronicity
            throw 'foo'
        }, 100);
    })()
}
catch(e){
    console.debug(e)
}

var reader = new FileReader();
                reader.onload = (function(f, isLast) {

Likely that's your problem right there - the FileReader probably calls onload asynchronously, at a time when your try/catch is no longer in scope. There may be a separate error handler function available on the FileReader interface, or you might need to move the try/catch into the anonymous function you're passing to onread().

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