Home > Abakada: Back to Basics > Basic Scripting > Data Entry Validation > Rounding a number to n decimals

Abakada ~ Back to Basics

---

If you always want to round to two decimals use a function like

function round2 ( n ) {
   n = Math.round ( n * 100 ) / 100;
   n = ( n + 0.001 ) + '';
   return n.substring ( 0, n.indexOf ( '.' ) + 3 );
}

which converts a given number into a string that is formatted as intended. Calling parseFloat on the result easily converts the output to a floating-point number, if needed. To check:

Now, using the above function plus the function below, we can format a number to a currency value:

function currency ( n, d ) {
   n = round2 ( n, d );
   for ( var i = n.indexOf ( '.' ) - 3; i > 0; i -= 3 )
      n = n.substring ( 0, i ) + ', ' + n.substring ( i );
   return n;
}

Note that the currency converter above expects a valid string value that can be converted to a number — specifically, can contain only digits and decimals, and not commas or other characters. If so, the function returns NaN ( not a number ).

In actual use, the value to convert is typically passed from user input in a field that has already been validated for numeric entries only.

If you need to round to a varying number of decimal places, use the below function instead.

function roundn ( n, d ) {
n = n - 0;
d = d == void ( 0 ) ? 2 : d
var f = Math.pow ( 10, d );
n = Math.round ( n * f ) / f;
n += Math.pow ( 10, - ( d + 1 ) );
n += '';
return d == 0 ? n.substring ( 0, n.indexOf ( '.' ) ) :
  n.substring ( 0, n.indexOf ( '.' ) + d + 1 );
}

which takes a number to round as the first argument, and an optional second argument to specify the number of decimals needed. To check:

Also note that starting with IE5.5 and NN6, there is a toFixed method for numbers which essentially does the above function.

var n = 2;
alert ( n.toFixed ( 2 ) ) 

If you would like to have the same functionality in IE4/5 and Netscape 4, this can be done by prototyping the Number object as follows:

if ( typeof ( Number ) !='undefined' &&
  typeof ( Number.prototype ) !='undefined' ) {
  if ( typeof ( Number.prototype.toFixed ) =='undefined' ) {
    function Number_toFixed ( d ) {
    var n = this
    d = d==void ( 0 ) ? 2 : d
    var f = Math.pow ( 10, d );
    n = Math.round ( n * f ) / f;
    n += Math.pow ( 10, - ( d + 1 ) );
    n += '';
    return d == 0 ? n.substring ( 0, n.indexOf ( '.' ) ) :
      n.substring ( 0, n.indexOf ( '.' ) + d + 1 );
     }
  Number.prototype.toFixed = Number_toFixed;
   }
}

You can then use toFixed in IE 4/5 and Netscape 4 in the same manner as in IE 5.5 and NN6.

Validating Against a Data Type ( Web Forms )   Web Forms Validation