function test() {
    alert("hello");
}

function hook(func_name, hook_func) {
    var hook_code = hook_func.toString();
    hook_code = hook_code.substring(hook_code.indexOf("{"));

    var f = eval(func_name + ".toString()");
    i = f.indexOf(func_name);
    f = f.substring(0, i) + f.substring(i + func_name.length);
    i = f.indexOf("{");
    f = f.substring(0, i + 1) + hook_code + f.substring(i + 2);
    var s = func_name + " = " + f;
    eval(s);
}

hook("test", function() {
    var s = "function intercepted!";
    for(var i = 0; i < arguments.length; i++)
        s += "\r\narg " + i + ": " + arguments[i];
    alert(s);
});
test(1, 2, 3);