Editing Bookmarklet (section)

== Concept ==

Web browsers use [[Uniform Resource Identifier|URI]]s for the <code>href</code> attribute of the <syntaxhighlight lang="HTML" inline><a></syntaxhighlight> tag and for bookmarks.  The URI scheme, such as <code>http</code> or <code>ftp</code>, and which generally specifies the [[Protocol (computing)|protocol]], determines the format of the rest of the string.  Browsers also implement <code>javascript:</code> URIs that to a parser is just like any other URI. The browser recognizes the specified <code>''javascript''</code> scheme and treats the rest of the string as a JavaScript program which is then executed.  The expression result, if any, is treated as the HTML [[source code]] for a new page displayed in place of the original.

The executing script has access to the current page, which it may inspect and change. If the script returns an undefined type (rather than, for example, a string), the browser will not load a new page, with the result that the script simply runs against the current page content. This permits changes such as in-place font size and color changes without a page reload.

An [[Immediately invoked function expression|immediately invoked function]] that returns no value or an expression preceded by the <code>void</code> operator will prevent the browser from attempting to parse the result of the evaluation as a snippet of HTML markup:
<syntaxhighlight lang="javascript" line="1" start="1">
javascript:(function(){
  //Statements returning a non-undefined type, e.g. assignments
})();
</syntaxhighlight>