Question

Is there any authoritative reference about the syntax and encoding of an URL for the pseudo-protocol javascript:? (I know it’s not very well considered, but anyway it’s useful for bookmarklets).

First, we know that standard URLs follow the syntax:

scheme://username:password@domain:port/path?query_string#anchor

but this format doesn’t seem to apply here. Indeed, it seems, it would be more correct to speak of URI instead of URL : here is listed the “unofficial” format javascript:{body}.

Now, then, which are the valid characters for such a URI, (what are the escape/unescape rules) when embedding in a HTML?

Specifically, if I have the code of a javascript function and I want to embed it in a javascript: URI, which are the escape rules to apply?

Of course one could escape every non alfanumeric character, but that would be overkill and make the code unreadable. I want to escape only the necessary characters.

Further, it’s clear that it would be bad to use some urlencode/urldecode routine pair (those are for query string values), we don’t want to decode ‘+’ to spaces, for example.

Answer 1

My findings, so far:

First, there are the rules for writing a valid HTML attribute value: but here the standard only requires (if the attribute value if enclosed in quotes) an arbitrary CDATA (actually a %URI, but HTML itself does not impose additional validation at its level: any CDATA will validate).

Some examples:

 <a href="javascript:alert('Hi!')">     (1)
 <a href="javascript:if(a > b && 1 < 0) alert(  b ? 'hi' : 'bye')">   (2)
 <a href="javascript:if(a&gt;b &amp;&&amp; 1 &lt; 0) alert( b ? 'hi' : 'bye')">  (3)

Example (1) is valid. But also example (2) is valid HTML 4.01 Strict. To make it valid XHTML we only need to escape the XML special characters < > & (example 3 is valid XHTML 1.0 Strict).

Now, is example (2) a valid javascript: URI ? I’m not sure, but I’d say it’s not.

From RFC 2396: an URI is subject to some addition restrictions and, in particular, the escape/unescape via %xx sequences. And some characters are always prohibited:
among them spaces and {}# .

The RFC also defines a subset of opaque URIs: those that do not have hierarchical components, and for which the separating charactes have no special meaning (for example, they dont have a ‘query string’, so the ? can be used as any non special character). I assume javascript: URIs should be considered among them.

This would imply that the valid characters inside the ‘body’ of a javascript: URI are

 a-zA-Z0-9 
 _|. !~*'();?:@&=+$,/-   
 %hh : (escape sequence, with two hexadecimal digits)

with the additional restriction that it can’t begin with /.
This stills leaves out some “important” ASCII characters, for example

{}#[]<>^\

Also % (because it’s used for escape sequences), double quotes " and (most important) all blanks.

In some respects, this seems quite permissive: it’s important to note that + is valid (and hence it should not be ‘unescaped’ when decoding, as a space).

But in other respects, it seems too restrictive. Braces and brackets, specially: I understand that they are normally used unescaped and browsers have no problems.

And what about spaces? As braces, they are disallowed by the RFC, but I see no problem in this kind of URI. However, I see that in most bookmarklets they are escaped as “%20”. Is there any (empirical or theorical) explanation for this?

I still don’t know if there are some standard functions to make this escape/unescape (in mainstream languages) or some sample code.