Restrict to 2 decimal places in keypress of a text box?

Question 1

You were almost there. Just check that there are no more than 2 characters after the decimal.

UPDATE 1 - check carat position to allow character insertion before the decimal.
UPDATE 2 - correct issue pointed out by ddlab's comment and only allow one dot.

 function validateFloatKeyPress(el, evt) {
    var charCode = (evt.which) ? evt.which : event.keyCode;
    var number = el.value.split('.');
    if (charCode != 46 && charCode > 31 && (charCode < 48 || charCode > 57)) {
        return false;
    }
    //just one dot (thanks ddlab)
    if(number.length>1 && charCode == 46){
         return false;
    }
    //get the carat position
    var caratPos = getSelectionStart(el);
    var dotPos = el.value.indexOf(".");
    if( caratPos > dotPos && dotPos>-1 && (number[1].length > 1)){
        return false;
    }
    return true;
}

//thanks: http://javascript.nwbox.com/cursor_position/
function getSelectionStart(o) {
    if (o.createTextRange) {
        var r = document.selection.createRange().duplicate()
        r.moveEnd('character', o.value.length)
        if (r.text == '') return o.value.length
        return o.value.lastIndexOf(r.text)
    } else return o.selectionStart
}

~~http://jsfiddle.net/S9G8C/1/~~
http://jsfiddle.net/S9G8C/203/

Question 2

Consider leveraging HTML5's Constraint Validation API. It doesn't necessarily prevent typing invalid values, but the field is marked invalid and it halts submission of the <form> (by default). I added the <output> to illustrate why the browser considers e.g. "1.100" a valid value (it sees the numeric value as "1.1").

<input id="n" type="number" step=".01">

var
  n = document.getElementById('n'),
  o = document.getElementById('o'),
  didInputN = function(e) {
    o.value = n.valueAsNumber;
  };

n.addEventListener('input', didInputN);

input:invalid {
  color: white;
  background-color: red;
}

<input id="n" type="number" step=".01">
<output id="o" for="n"></output>

Philosophically, you might consider this a more usable approach as it allows the user to paste an invalid entry and edit it directly in the field.

Question 3

You can do it by another way with onchange event, to not restrict to user to type, rather just convert number after typing, to make uniform, like this,

function validateFloatKeyPress(el) {
    var v = parseFloat(el.value);
    el.value = (isNaN(v)) ? '' : v.toFixed(2);
}

<input id="aninput" type="text" onchange="validateFloatKeyPress(this);" />

45.846 should be 45.85 but in your code user needed to convert their-self and then they will type 45.85 directly

Question 4

1.)No multiple decimals points.
2.)Two numbers after decimal point.
3.)Allow only Numbers and one decimal point(.).

This will help.jsFiddle

Question 5

 function decimalValidation(el, evt) {
       var charCode = (evt.which) ? evt.which : event.keyCode;
       var number = el.value.split('.');
         if(charCode == 8) {
          return true;
         }
        if (charCode != 46 && charCode > 31 && (charCode < 48 || charCode > 57)) {
            return false;
        }
        //just one dot
        if(number.length>1 && charCode == 46){
             return false;
        }
        //get the carat position
        var caratPos = getSelectionStart(el);
        var dotPos = el.value.indexOf(".");
        if( caratPos > dotPos && dotPos>-1 && (number[1].length > 1)){
            return false;
        }
        return true;
    }

function getSelectionStart(o) {
    return o.selectionStart
}

Hi @webvitaly The above code will work in IE too please check And backspace after decimals not working in Mozilla i updated my answer.

Question 6

this code is very complet, I change "." to ",":

can't "," in begin

can't write more ","

<script type="text/javascript">

    function isNumberKey(evt, el) {
        var charCode = (evt.which) ? evt.which : event.keyCode;
        var number = el.value.split(',');
        var caracter = el.value;

        if (charCode != 44 && charCode > 31 && (charCode < 48 || charCode > 57)) {
            return false;
        }

        if (charCode == 44 && caracter == "") {
            return false;
        }

        if (charCode == 44 && caracter.indexOf(",") != -1) {
            return false;
        }

        //get the carat position
        var caratPos = getSelectionStart(el);
        var dotPos = el.value.indexOf(",");
        if (caratPos > dotPos && dotPos > -1 && (number[1].length > 1)) {
            return false;
        }
        return true;
    }

    function getSelectionStart(o) {
        if (o.createTextRange) {
            var r = document.selection.createRange().duplicate()
            r.moveEnd('character', o.value.length)
            if (r.text == '') return o.value.length
            return o.value.lastIndexOf(r.text)
        } else return o.selectionStart
    }

</script>

Question 7

My problem was that I need it to show an error message in real time if the user is allowed only 2 decimals:

value = parseFloat(valueFromInput);
parseFloat(value.toFixed(2)) !== value // condition to check

The above code worked for me..toFixed converts the float to a string wit only 2 decimals and I have to convert back to float to check with the initial value if are the same.

P.S. And before this condition you should check if the value is NaN.