How can I add an unremovable prefix to an HTML input field?

Question

Using jQuery, how can I add a default value of http:// into an input field that can’t be removed, but that still allows you to type a URL after it?

Defa

Answer 1

UPDATE: Guillaumesm's answer to Ecmascript 5:

Form.prototype.set_HttpPrefix = function (e) {
    if (e.data.$url.val().length < 7) {
        e.data.$url.val('http://')

    }    
    else if (e.data.$url.val().indexOf('http://') !== 0) {
        e.data.$url.val('http://' + String.fromCharCode(e.which)) 
    }
}