Bookmarklets: The "javascript:" Protocol - www.docjavascript.com

The JavaScript statement used in a javascript: URL should not return any value. For example, the alert() method doesn't return a value; in other words, it returns undefined. If the statement returns undefined, the browser simply executes it. However, if it returns an explicit value, the browser loads a new page, with the javascript: URL in the Location bar, and the returned value in the body of the page.

When taking advantage of the javascript: protocol, we must make sure the statement doesn't return any value. Therefore, we need to apply the void operator if the statement returns an actual value. Here's an example:

<A HREF="javascript:void(str='Doc JavaScript')">

The void operator evaluates its expression, and returns undefined. The parentheses surrounding the expression are optional, but they are important when writing compact, one-line scripts. If you're not sure if a statement returns a value, use the void operator to stay on the safe side.

When inserting several statements in a single javascript: URL, all value-returning statements require a void operator. Take a look at the following example:

<A HREF="javascript:void(a=1);void(b=prompt('Enter a number:','99'))">

Note that the entire javascript: URL should be as compact as possible. Long javascript: URLs tend to generate errors or crash the browser. We suggest that you stick to a maximum of about 500 characters.