FormValidator.prototype.validate = function validateForm() {
    this.errors = {};
    for (var fieldName in this.fields) {
        var field = this.fields[fieldName];

        if (field.hasOwnProperty("required") && field.required && field.elem.value.length == 0) {
            this.addError(fieldName, "This field is required.");
            break;
        }

        if (field.hasOwnProperty("minLength") && field.elem.value.length < field.minLength) {
            this.addError(fieldName, "Input length should not be less than  " + field.minLength + " characters.");
            break;
        }

        if (field.hasOwnProperty("maxLength") && field.elem.value.length > field.maxLength) {
            this.addError(fieldName, "Input length should not be greater than" + field.maxLength + " characters.");
            break;
        }

        if (field.hasOwnProperty("ajax")) {
            // FormValidator can't possibly know what the call will return, so we can't add the error here
            // it has to be done manually
            field.ajax(this, field, fieldName);
        }
    }
    if (this.errors.length != 0) {
        // warn the user
        console.log(this.errors);
    }
};

var fv = new FormValidator(document.forms[0]);

fv.addField("login_name", {
    type      : "text",
    minLength : 4,
    maxLength : 32,
    required  : true,
    ajax      : function (fv, field, fieldName) {
        ajax("http://localhost/surec/scripts/user_check.php?field=login_name&value=" + field.elem.value, {
            success : function () {
                var response = JSON.parse(this.response);
                // manually adding the error
                if (!response.error && response.exists) {
                    fv.addError(fieldName, "This username is taken.");
                }
            },
            // async: false,
        });
    },
});

// called on form submit
// fv.validate();