JavaScript function to validate a text field on a form which is to contain an integer,without using any built functions like IsNaN

I want to write a JavaScript function to validate a text field on a form which is to contain an integer, without using any built functions like IsNaN an so fourth.  If the text field contains a valid number then the JavaScript function can return “true”, otherwise it returns “false” as well as displaying a popup error message.
leestottAsked:
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Michel PlungjanIT ExpertCommented:
And Why no built-in functions?
nums = "0123456789"
function isInteger(str) {
  if (!str) return false; // empty
  testStr = ""+str
  for (i=0;i<testStr.length;i++) {
    if (nums.indexOf(str.charAt(i)) ==-1) return false
  }
  return true
}
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Michel PlungjanIT ExpertCommented:
With popup - but still using the built-in indexOf and charAt
function isInteger(str) {
  if (!str) return false; // empty
  testStr = ""+str
  for (i=0;i<testStr.length;i++) {
    if (nums.indexOf(str.charAt(i)) ==-1) {
      alert(str+' is not a number')
      return false
    }
  }
  return true
}
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GwynforWebCommented:
Try this (what exactly do you mean by no special functions?)

<form name='f1'>
<input type="text" name='t1'>
<input type="button" name='t2' value='check value' onclick='check(t1.value)'>
</form>

<script>
function check(value)
{
 if (/^\d+$/.test(value))
      alert(value+" is valid")
 else
      alert(value+" is invalid")
}
</script>


I you want to allow numbers like 123. and 123.00 then change
/^\d+$/.test(value)   to   /^\d+((\.0*))*$/.test(value)

(note regular expression as used here can be used in many laguages other than javascript)

another alternative is this

<form name='f1'>
<input type="text" name='t1'>
<input type="button" name='t2' value='check value' onclick='check(t1.value)'>
</form>

<script>
function check(value)
{
 if (value-Math.floor(value) == 0)
      alert(value+" is valid")
 else
      alert(value+" is invalid")
}
</script>

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Michel PlungjanIT ExpertCommented:
Riiight... RegExps are less compatible than isNaN - ;)
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ZontarCommented:
If you're concerned about compatibility, parseInt() is JS 1.0 (NN2+) and isNan() is JS1.1 (NS3+/MSIE3+) whereas regular expressions are JS 1.2 (NS4+/MSIE4+). And parseInt() and isNaN() are much simpler and less resource-intensive.

This isn't a homework assignment, is it?
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GwynforWebCommented:
"regular expressions as used here can be used in many laguages other than javascript" is true as "isNaN is not used in many other languages" , browser compatibility is a different issue.
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leestottAuthor Commented:
The reason is I have a number of form on our web server which is amount box and what to ensure noone enter number and not intergers such as £100,234 they can only enter a real . the reason for asking for a regex is due to the site mainly being built in asp.net so if I have a regex easily port to vb etc.

Thanks
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Michel PlungjanIT ExpertCommented:
You did not ask for a regexp... If you had I would not have given you the suggestion I gave.
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