Format Phone Numbers onblur

HTML

<input type="text" name="test" onblur="formatPhone(this);" />

JavaScript

function formatPhone(obj) {
    var numbers = obj.value,
        char = {2:' (',5:') ',8:' - '};
    obj.value = '';
    for (var i = 0; i < numbers.length; i++) {
        obj.value += (char[i]||'') + numbers[i];
    }
}