How to delete default values in input fields on focus while keeping values typed by the user with jQuery

shinywhitebox.js

    // Detetes default value in input fields but not user inserted text
    default_values = {};
    $email = $('input#email');

    $email.each(function () {
        var index = $(this).attr('name');
        var value = $(this).val();
        default_values[index] = value;
    });

    $email.focus(function() {
        if ($(this).val() === default_values[$(this).attr('name')]) {
            $(this).val('');
        };
    });