Frontend number formating - thousands separators and rounding

It's good for a backend developer to do some JS scripting from time to time. Usually this scripting considers the user interface or other presentation mechanisms. Everybody knows that a good user interface has a great impact for attracting people to use your service.

Let's skip positioning/css stuff and move along to data presentation. Usually there is a difference between the data we actually process and the data shown to users. So we have a variable containing the cost of some imaginary service, i.e. 4231.1578. This looks just like an old-school float representation - and in fact it is. So whats wrong with the users?... oops... I mean why is  this representation not friendly to users?

Firstly, a common decimal delimiter for European countries is a comma, not a dot - we should respect it.

Secondly, humans do not process text the way machines - the larger the presented amount, the longer it takes to evaluate its value, or even the order of magnitude. Thats why you should provide thousands separators, all in all we get something like this: 4.231,1578

Now all it takes round the value a bit, we don't need so many decimal places. Below the code that converts your js floats to a formatted and readable string (val  is the float value, while dec_places is the number of decimal places).

1 function round_repr(val, dec_places){

2     var tmp = Math.round((val*Math.pow(10, dec_places))).toString();

3     tmp = tmp.slice(0, tmp.length-dec_places) + ',' + tmp.slice(tmp.length-dec_places, tmp.length);

4     var int_length = tmp.length - dec_places - 1 - 3;

5     var i = int_length;

6     for (i=int_length; i>0; i-=3){

7         tmp = tmp.slice(0, i) + "." + tmp.slice(i,tmp.length);

8     }

9    return tmp;

10   

11 }

~KR