Why Do Some Websites Split `` Tags Using `document.write()`?

Splitting <script> Tags in document.write()</script>

Splitting <script> and </script> tags using document.write() is a technique employed by some websites to dynamically add scripts to a webpage.

Why is Splitting Necessary?

In HTML, <script> tags enclose JavaScript code. When the browser encounters a <script> tag, it interprets the code within and executes it. However, if the <script> tag is split into two parts and written with document.write(), it prevents premature termination of the script block.</script>

Browser Behavior with CDATA

In HTML, <script> tags use a CDATA (Character Data) section for their content. CDATA blocks are supposed to end when they encounter an end-tag open delimiter, such as <. However, browsers typically only terminate CDATA script blocks on an actual </script> close-tag.

Example

For instance, the following code from Amazon demonstrates the technique:

<script type="text/javascript">
  if (typeof window['jQuery'] == 'undefined') document.write('<scr' + 'ipt type="text/javascript" src="http://z-ecx.images-amazon.com/images/G/01/javascripts/lib/jquery/jquery-1.2.6.pack._V265113567_.js">' + 'ript>');
</script>

Alternative Approaches

In XHTML, scripts must be escaped to prevent confusion. A better approach is to use a hexadecimal character sequence to write the script element, as seen below:

<script type="text/javascript">
    document.write('\x3Cscript type="text/javascript" src="foo.js">\x3C/script>');
</script>

By following these guidelines, developers can effectively incorporate dynamic scripts into their webpages while maintaining compatibility with different browsers and HTML standards.

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