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posts by Jesse Hallett

Monkey patching document.write()

This is one of the crazier workarounds that I have implemented. I was working on a web page that embeds third-party widgets. The widgets are drawn in the page document - they do not get their own frames. And sometimes the widgets are redrawn after page load.

We had a problem with one widget invoking document.write(). In case you are not familiar with it, if that method is called while the page is rendering it inserts content into the DOM immediately after the script tag in which the call is made. But if document.write() is called after page rendering is complete it erases the entire DOM. When this widget was redrawn after page load it would kill the whole page.

The workaround we went with was to disable document.write() after page load by replacing it with a wrapper that checks whether the jQuery ready event has fired.

(function() {
    var originalWrite = document.write;
    document.write = function() {
        if (typeof jQuery !== 'undefined' && jQuery.isReady) {
            if (typeof console !== 'undefined' && console.warn) {
                console.warn("document.write called after page load");
            }
        }
        else {
            // In IE before version 8 `document.write()` does not
            // implement Function methods, like `apply()`.
            return Function.prototype.apply.call(
                originalWrite, document, arguments
            );
        }
    }
})();

The new implementation checks the value of jQuery.isReady and delegates to the original document.write() implementation if the page is not finished rendering yet. Otherwise it does nothing other than to output a warning message.

Disabling document.write() means that the problematic widget will not be fully functional if it is redrawn after page load. It happens that in the case of this app that is ok. The redrawn widget is only used as a preview when editing widget layouts.

A particular problem came up with IE compatibility. I wanted to use the apply method that is implemented by all functions in JavaScript to invoke the original document.write() implementation, like this:

return originalWrite.apply(document, arguments);

But in older versions of Internet Explorer, document.write() is not really a function. There are a lot of examples in IE of native API methods and properties that do not behave like regular JavaScript values. For example if you pass too many arguments to a DOM API method in old IE you will get an exception. Normal JavaScript functions just silently ignore extra arguments. If you look at the value of typeof document.write the result is not "function". What is particularly problematic in this case is that document.write does not implement call or apply.

Fortunately I found that the Function prototype does implement both call and apply and furthermore you can borrow those methods to use on function-like values like document.write. call and apply are themselves real function values - so call and apply both implement call and apply.

typeof Function.prototype.apply.apply.apply  // evaluates to 'function'
typeof Function.prototype.apply.call.call  // evaluates to 'function'

In the workaround above I applied apply to document.write by taking the Function.prototype.apply value and using its call method. So this expression,

Function.prototype.apply.call(originalWrite, document, arguments);

is equivalent to this one,

originalWrite.apply(document, arguments);

Except that the first version works in IE7.

If you find this difficult to follow, you are not alone.

We have had this workaround in our code for a couple of years now. So far it is working nicely.

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