how to format input box text as I am typing it

Pure JS (Sans jQuery):

var fnf = document.getElementById("formattedNumberField");
fnf.addEventListener('keyup', function(evt){
    var n = parseInt(this.value.replace(/\D/g,''),10);
    fnf.value = n.toLocaleString();
}, false);

Native JS Example Fiddle

With jQuery:

$("#formattedNumberField").on('keyup', function(){
    var n = parseInt($(this).val().replace(/\D/g,''),10);
    $(this).val(n.toLocaleString());
    //do something else as per updated question
    myFunc(); //call another function too
});

With jQuery Example Fiddle

To allow decimals:

$("#formattedNumberField").on('keyup', function(evt){
    if (evt.which != 110 ){//not a fullstop
        var n = parseFloat($(this).val().replace(/\,/g,''),10);
        $(this).val(n.toLocaleString());
    }
});

Obligatory Example

With some code bits of other answers I created the following working example. Code is without jquery, but a little bit longer than some other examples. But it takes care of a lot of the problems that others have.

http://jsfiddle.net/kcz4a2ca/10/

Code:

var input = document.getElementById('my_textbox');
var currentValue;

input.addEventListener('input', function(event) {
  var cursorPosition = getCaretPosition(input);
  var valueBefore = input.value;
    var lengthBefore = input.value.length;
  var specialCharsBefore = getSpecialCharsOnSides(input.value);
  var number = removeThousandSeparators(input.value);

  if (input.value == '') {
    return;
  }

  input.value = formatNumber(number.replace(getCommaSeparator(), '.'));

    // if deleting the comma, delete it correctly
  if (currentValue == input.value && currentValue == valueBefore.substr(0, cursorPosition) + getThousandSeparator() + valueBefore.substr(cursorPosition)) {
    input.value = formatNumber(removeThousandSeparators(valueBefore.substr(0, cursorPosition-1) + valueBefore.substr(cursorPosition)));
    cursorPosition--;
  }

  // if entering comma for separation, leave it in there (as well support .000)
  var commaSeparator = getCommaSeparator();
    if (valueBefore.endsWith(commaSeparator) || valueBefore.endsWith(commaSeparator+'0') || valueBefore.endsWith(commaSeparator+'00') || valueBefore.endsWith(commaSeparator+'000')) {
    input.value = input.value + valueBefore.substring(valueBefore.indexOf(commaSeparator));
  }

  // move cursor correctly if thousand separator got added or removed
  var specialCharsAfter = getSpecialCharsOnSides(input.value);
  if (specialCharsBefore[0] < specialCharsAfter[0]) {
        cursorPosition += specialCharsAfter[0] - specialCharsBefore[0];
  } else if (specialCharsBefore[0] > specialCharsAfter[0]) {
        cursorPosition -= specialCharsBefore[0] - specialCharsAfter[0];
  }
  setCaretPosition(input, cursorPosition);

  currentValue = input.value;
});

function getSpecialCharsOnSides(x, cursorPosition) {
    var specialCharsLeft = x.substring(0, cursorPosition).replace(/[0-9]/g, '').length;
  var specialCharsRight = x.substring(cursorPosition).replace(/[0-9]/g, '').length;
  return [specialCharsLeft, specialCharsRight]
}

function formatNumber(x) {
   return getNumberFormat().format(x);
}

function removeThousandSeparators(x) {
  return x.toString().replace(new RegExp(escapeRegExp(getThousandSeparator()), 'g'), "");
}

function getThousandSeparator() {
  return getNumberFormat().format('1000').replace(/[0-9]/g, '')[0];
}

function getCommaSeparator() {
  return getNumberFormat().format('0.01').replace(/[0-9]/g, '')[0];
}

function getNumberFormat() {
    return new Intl.NumberFormat();
}

/* From: http://stackoverflow.com/a/6969486/496992 */
function escapeRegExp(str) {
  return str.replace(/[\-\[\]\/\{\}\(\)\*\+\?\.\\\^\$\|]/g, "\\$&");
}

/*
** Returns the caret (cursor) position of the specified text field.
** Return value range is 0-oField.value.length.
** From: http://stackoverflow.com/a/2897229/496992
*/
function getCaretPosition (oField) {
  // Initialize
  var iCaretPos = 0;

  // IE Support
  if (document.selection) {

    // Set focus on the element
    oField.focus();

    // To get cursor position, get empty selection range
    var oSel = document.selection.createRange();

    // Move selection start to 0 position
    oSel.moveStart('character', -oField.value.length);

    // The caret position is selection length
    iCaretPos = oSel.text.length;
  }

  // Firefox support
  else if (oField.selectionStart || oField.selectionStart == '0')
    iCaretPos = oField.selectionStart;

  // Return results
  return iCaretPos;
}

/* From: http://stackoverflow.com/a/512542/496992 */
function setCaretPosition(elem, caretPos) {
    if(elem != null) {
        if(elem.createTextRange) {
            var range = elem.createTextRange();
            range.move('character', caretPos);
            range.select();
        }
        else {
            if(elem.selectionStart) {
                elem.focus();
                elem.setSelectionRange(caretPos, caretPos);
            }
            else
                elem.focus();
        }
    }
}

Try this plugin it works pretty nice http://robinherbots.github.io/jquery.inputmask/