Question

I am validating text input on a html form, and would like to focus and select the text if it is invalid.

If my validation function looks like (just an example):

jQuery.fn.DateHelper = function(text){
    return this.each(function(){
    //Make sure we're dealing with text-based form fields
    if(this.type != 'text')
    return;

    // call the processNumber function when the onBlur event is fired and there is a value in the field.
    $(this).blur(function() {
        if (this.value != '')                 
            processNUmber(this);
        });

    function processNumber( txtField ) {
        if (!isNan(txtField ))  {
            alert("not a number");
            this.focus(); // this does not work.
            // how to select text?     
        }

    }

};

Answer 1

In your function you pass the txtField parameter, use it instead of this within the processNumber function:

function processNumber( txtField ) {
    if (!isNan(txtField ))  {
        alert("not a number");
        txtField.focus();   
    }
}

Also you are declaring the processNumber function inside the .each() loop in the jQuery.fn.DateHelper function which doesn’t have to be the case, you can declare that function outside the .each() and it will be accessible to any element on which this plugin is run.

UPDATE

Your code functions however there are several syntactical errors:

jQuery.fn.DateHelper = function(text){
    return this.each(function(){
        //Make sure we're dealing with text-based form fields
        if(this.type != 'text') return;

        // call the processNumber function when the onBlur event is fired and there is a value in the field.
        $(this).blur(function() {
            if (this.value != '') processNumber(this);//you were calling the processNUmber function, which doesn't exist
        }).focus(function () {
            this.select();
        });

        function processNumber( txtField ) {
            //isNan is broken: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/isNaN
            //also txtField is a DOM element, not a string
            txtField.value = txtField.value.replace(/[^0-9.]/g, "");
            if (txtField.value.length > 0)  {
                alert("not a number");
                txtField.focus();    
            }
        }
    });//this was omitted
};

Here is a demo: http://jsfiddle.net/uQ45E/6/