Javascript not-well formed error in for loop

Question

I copied this code from an example. I've read it 100 times.

Array.prototype.map = function(fn) {
    var r = [];
    var l = this.length;
    for(var i = 0; i < l; i++) {
        r.push(fn(this[i]));
    }
    return r;
};

Why does Firefox say:

not well-formed
file:///some/path.html                         Line: 5
    for(var i = 0; i < l; i++) {
    -------------------^

UPDATE

The error is only shown when Firebug is turned on for the page.

Answer 1

You are using Javascript code in an HTML page claiming to be fully XHTML-compliant. Therefore, the < character cannot appear in the Javascript, as it would be interpreted as the beginning of an XHTML tag.

There are a number of ways to fix this.

You could change the DOCTYPE and make it not XHTML.
You could enclose the Javascript in a <![CDATA[ section.
You could move the Javascript into a separate .js file.
You could escape every occurrence of < with &lt and every & with &. I do not recommend this option; it'll make your code unreadable and will almost definitely not work in IE.

Answer 2

Likely your error isn't in this code, but something above it trickling errors down. So, instead of finding an error in this code, look above for malformed HTML or javascript that could be causing this error instead.