Why does decodeURIComponent('%') lock up my browser?

Question

I was just testing something with AJAX and I found that on success if I alert

alert(decodeURI('%'));

or

alert(encodeURIComponent('%'));

the browser errors out with the following code.

$.ajax({
   type: "POST",
   url: "some.php",
   data: "",
   success: function(html){
         alert(decodeURIComponent('%'));
//           alert(decodeURI('%'));
   }
 });

If I use any other string it works just fine.
Is it something that I missed?

Answer 1

Recently a decodeURIComponent in my code tripped over the ampersand % and googling led me to this question.

Here's the function I use to handle % which is shorter than the version of Ilia:

function decodeURIComponentSafe(s) {
    if (!s) {
        return s;
    }
    return decodeURIComponent(s.replace(/%(?![0-9][0-9a-fA-F]+)/g, '%25'));
}

It

• returns the input value unchanged if input is empty
• replaces every % NOT followed by a two-digit (hex) number with %25
• returns the decoded string

It also works with the other samples around here:

• decodeURIComponentSafe("%%20Visitors") // % Visitors
• decodeURIComponentSafe("%Directory%20Name%") // %Directory Name%
• decodeURIComponentSafe("%") // %
• decodeURIComponentSafe("%1") // %1
• decodeURIComponentSafe("%3F") // ?

Answer 2

Chrome barfs when trying from the console. It gives an URIError: URI malformed. The % is an escape character, it can't be on its own.