Josh Stodola Josh Stodola - 2 months ago 7
Javascript Question

How to avoid global variables in JavaScript?

We all know that global variables are anything but best practice. But there are several instances when it is difficult to code without them. What techniques do you use to avoid the use of global variables?

For example, given the following scenario, how would you not use a global variable?

JavaScript code:

var uploadCount = 0;

window.onload = function() {
var frm = document.forms[0];

frm.target = "postMe";
frm.onsubmit = function() {
startUpload();
return false;
}
}

function startUpload() {
var fil = document.getElementById("FileUpload" + uploadCount);

if (!fil || fil.value.length == 0) {
alert("Finished!");
document.forms[0].reset();
return;
}

disableAllFileInputs();
fil.disabled = false;
alert("Uploading file " + uploadCount);
document.forms[0].submit();
}


Relevant markup:

<iframe src="test.htm" name="postHere" id="postHere"
onload="uploadCount++; if(uploadCount > 1) startUpload();"></iframe>

<!-- MUST use inline JavaScript here for onload event
to fire after each form submission. -->


This code comes from a web form with multiple
<input type="file">
. It uploads the files one at a time to prevent huge requests. It does this by POSTing to the iframe, waiting for the response which fires the iframe onload, and then triggers the next submission.

You don't have to answer this example specifically, I am just providing it for reference to a situation in which I am unable to think of a way to avoid global variables.

Answer

The easiest way is to wrap your code in a closure and manually expose only those variables you need globally to the global scope:

(function() {
    // Your code here

    // Expose to global
    window['varName'] = varName;
})();

To address Crescent Fresh's comment: in order to remove global variables from the scenario entirely, the developer would need to change a number of things assumed in the question. It would look a lot more like this:

Javascript:

(function() {
    var addEvent = function(element, type, method) {
        if('addEventListener' in element) {
            element.addEventListener(type, method, false);
        } else if('attachEvent' in element) {
            element.attachEvent('on' + type, method);

        // If addEventListener and attachEvent are both unavailable,
        // use inline events. This should never happen.
        } else if('on' + type in element) {
            // If a previous inline event exists, preserve it. This isn't
            // tested, it may eat your baby
            var oldMethod = element['on' + type],
                newMethod = function(e) {
                    oldMethod(e);
                    newMethod(e);
                };
        } else {
            element['on' + type] = method;
        }
    },
        uploadCount = 0,
        startUpload = function() {
            var fil = document.getElementById("FileUpload" + uploadCount);

            if(!fil || fil.value.length == 0) {    
                alert("Finished!");
                document.forms[0].reset();
                return;
            }

            disableAllFileInputs();
            fil.disabled = false;
            alert("Uploading file " + uploadCount);
            document.forms[0].submit();
        };

    addEvent(window, 'load', function() {
        var frm = document.forms[0];

        frm.target = "postMe";
        addEvent(frm, 'submit', function() {
            startUpload();
            return false;
        });
    });

    var iframe = document.getElementById('postHere');
    addEvent(iframe, 'load', function() {
        uploadCount++;
        if(uploadCount > 1) {
            startUpload();
        }
    });

})();

HTML:

<iframe src="test.htm" name="postHere" id="postHere"></iframe>

You don't need an inline event handler on the <iframe>, it will still fire on each load with this code.

Regarding the load event

Here is a test case demonstrating that you don't need an inline onload event. This depends on referencing a file (/emptypage.php) on the same server, otherwise you should be able to just paste this into a page and run it.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
    <title>untitled</title>
</head>
<body>
    <script type="text/javascript" charset="utf-8">
        (function() {
            var addEvent = function(element, type, method) {
                if('addEventListener' in element) {
                    element.addEventListener(type, method, false);
                } else if('attachEvent' in element) {
                    element.attachEvent('on' + type, method);

                    // If addEventListener and attachEvent are both unavailable,
                    // use inline events. This should never happen.
                } else if('on' + type in element) {
                    // If a previous inline event exists, preserve it. This isn't
                    // tested, it may eat your baby
                    var oldMethod = element['on' + type],
                    newMethod = function(e) {
                        oldMethod(e);
                        newMethod(e);
                    };
                } else {
                    element['on' + type] = method;
                }
            };

            // Work around IE 6/7 bug where form submission targets
            // a new window instead of the iframe. SO suggestion here:
            // http://stackoverflow.com/q/875650
            var iframe;
            try {
                iframe = document.createElement('<iframe name="postHere">');
            } catch (e) {
                iframe = document.createElement('iframe');
                iframe.name = 'postHere';
            }

            iframe.name = 'postHere';
            iframe.id = 'postHere';
            iframe.src = '/emptypage.php';
            addEvent(iframe, 'load', function() {
                alert('iframe load');
            });

            document.body.appendChild(iframe);

            var form = document.createElement('form');
            form.target = 'postHere';
            form.action = '/emptypage.php';
            var submit = document.createElement('input');
            submit.type = 'submit';
            submit.value = 'Submit';

            form.appendChild(submit);

            document.body.appendChild(form);
        })();
    </script>
</body>
</html>

The alert fires every time I click the submit button in Safari, Firefox, IE 6, 7 and 8.