throw | NN 6 IE 5 ECMA 3
Triggers an exception condition, passing a value along with the exception. Although the value you pass can be a simple string, ideally you should pass an instance of the JavaScript Error object filled with sufficient information for a catch statement to act intelligently on the error. A throw statement must be enclosed in the try portion of a try-catch construction.
Example
    function processNumber(inputField) {
        try {
            var inpVal = parseInt(inputField.value, 10);
            if (isNaN(inpVal)) {
                var msg = "Please enter a number only.";
                var err = new Error(msg);
                if (!err.message) {
                    err.message = msg;
                }
                throw err;
            }
            // process number
        }
        catch (e) {
            alert(e.message);
            inputField.focus( );
            inputField.select( );
        }
    }