1/31/2011 Modifying HTML strings using jQuery

jQuery makes it very easy to modify a DOM tree.  For example, to strip all hyperlinks (<a> tags) from an element, we can write (demo)

$(...).find('a[href]')
      .replaceWith(function() { return this.childNodes });

After getting used to this, one might want to use jQuery to modify HTML contained in a string.  Here, however, the naïve approach does not work:

var htmlSource = ...;
$(htmlSource).find('a[href]')
      .replaceWith(function() { return this.childNodes });

This code tries to remove all <a> tags from the HTML contained in the htmlSource string.  However, what it actually does is create a detached DOM tree containing the new elements, strip all <a> tags in those elements, and throw the whole thing away.  It doesn’t modify the original string.  In fact, since the  $ function only takes a reference to an immutable string, this approach cannot modify the original string.

Instead, you need to retrieve the source from the DOM tree after modifying it, then assign that source back to the variable. 

There is an additional subtlety with this approach.  jQuery cannot return the complete HTML source for a collection of elements.  Therefore, it is also necessary to wrap the HTML in a dummy element (typically a <div>).   One can then call .html() to get the innerHTML of the dummy element, which will contain exactly the desired content

This also eliminates the distinction between root-level elements and nested elements.  If the original HTML string contains root-level <a> elements (which aren’t nested in other tags), writing $(htmlSource).find('a') won’t find them, since .find() only searches the descendants of the elements in the jQuery object.  By wrapping the HTML in a dummy element, all of the elements in the original content become descendants, and can be returned by .find().

Here, therefore, is the correct way to modify an HTML string using jQuery:

var htmlSource = ...;
var tree = $("<div>" + htmlSource + "</div>");

tree.find('a[href]')
    .replaceWith(function() { return this.childNodes });

htmlSource = tree.html();