Allow only numeric input inside text field

Javascript only numeric

document.getElementsByTagName('input')[0].onkeypress = function(e) {
  e = e || event
  var chr = getChar(e)
  
  if (!isNumeric(chr) && chr !== null) {
    return false
  }
}
// helper functions
function isNumeric(val) {
  return val !== "NaN" && (+val)+'' === val + ''
}

function getChar(event) {
  if (event.which == null) {
    return String.fromCharCode(event.keyCode) // IE
  } else if (event.which!=0 && event.charCode!=0) {
    return String.fromCharCode(event.which)   // the rest
  } else {
    return null // special key
  }
}