Why is document.write considered a “bad practice”?

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I know document.write is considered bad practice; and I’m hoping to compile a list of reasons to submit to a 3rd party vendor as to why they shouldn’t use document.write in implementations of their analytics code.

Please include your reason for claiming document.write as a bad practice below.

First answer

A few of the more serious problems:

  • document.write (henceforth DW) does not work in XHTML

  • DW does not directly modify the DOM, preventing further manipulation (trying to find evidence of this, but it’s at best situational)

  • DW executed after the page has finished loading will overwrite the page, or write a new page, or not work

  • DW executes where encountered: it cannot inject at a given node point

  • DW is effectively writing serialised text which is not the way the DOM works conceptually, and is an easy way to create bugs (.innerHTML has the same problem)

Far better to use the safe and DOM friendly DOM manipulation methods

Second answer

Another legitimate use of document.write comes from the HTML5 Boilerplate index.html example.

<!-- Grab Google CDN's jQuery, with a protocol relative URL; fall back to local if offline -->
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.6.3/jquery.min.js"></script>
<script>window.jQuery || document.write('<script src="js/libs/jquery-1.6.3.min.js"><\/script>')</script>

I’ve also seen the same technique for using the json2.js JSON parse/stringify polyfill (needed by IE7 and below).

<script>window.JSON || document.write('<script src="json2.js"><\/script>')</script>

Third answer

It can block your page

document.write only works while the page is loading; If you call it after the page is done loading, it will overwrite the whole page.

This effectively means you have to call it from an inline script block – And that will prevent the browser from processing parts of the page that follow. Scripts and Images will not be downloaded until the writing block is finished.

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