javascript - Round value using selection of drop down menu / radio button -

this consists of 2 problems. first problem rounding numbers decimal point. second problem connecting drop down menu right decimal.

first code:

<style>   span{font-style:italic;}   span{color:green;} </style>  <script>     function calcul(){             var sales = parsefloat(document.getelementbyid("sales").value);             var opexp = parsefloat(document.getelementbyid("opexp").value);             var taxall = parsefloat(document.getelementbyid("taxall").value);             var depre = parsefloat(document.getelementbyid("depre").value);             var divid = parsefloat(document.getelementbyid("divid").value);             var taxr = parsefloat(document.getelementbyid("taxr").value);             //gp = gross profit             var gp = sales - opexp;             //taxinc = taxable income             var taxinc = gp + taxall;             //netinc = net income             var netinc = taxinc - ((taxr / 100) * taxinc);             document.getelementbyid("netincome").innerhtml=              taxinc - ((taxr / 100) * taxinc);             //atre = addition retained earnings             document.getelementbyid("atre").innerhtml = netinc - divid;         } </script>  <form action="" id="nothing"> in 2007 british building firm balfour betty plc had sales of  <input type="text" id="sales" maxlength="6" size="6">  million, total operating expenses of  <input type="text" id="opexp" maxlength="6" size="6">  million, tax allowance (rebate) of  <input type="text" id="taxall" maxlength="6" size="6">  million because of past losses, ,  <input type="text" id="depre" maxlength="6" size="6">  depreciation. <strong>what net income of firm?</strong><br /> <br /> balfour betty plc paid out <input type="text" id="divid" maxlength="6" size="6"> million  in cash dividends. <strong>what addition retained earnings?</strong><br /> <br /> tax rate <input type="text" id="taxr"  maxlength="6" size="6"> %.<br /> <br />   <input type="button" value="calculate" id="but" onclick="calcul()" /><br /> <br /> </form>  <strong>the net income of balfour betty plc </strong><span id="netincome">xxx</span> <strong> million</strong><br /> <br /> <strong>the addition retained earnings of balfour betty plc </strong><span id="atre"> xxx</span><strong> million</strong> <br /> <br /> 

the first problem: rounding numbers. following answer: <span id="netincome"> needs rounded dynamically. i've tried add new variable called rnetinc , add following equation rnetinc = netinc.tofixed(4), gave me 2 decimals, , after refresh doesn't work anymore. best way round answer n decimals?

the second problem 1 don't know if it's possible. have in mind following: dropdown menu

<select>     <option value"1">1 decimal</option>     <option value"2">2 decimals</option>     <option value"3">3 decimals</option> </select>  

so, want when click n decimal, answer change n decimal. complex situation, 1 need.

since know (very) basics of javascript, using google cannot find answer. can me on right track (if it's possible)? in advance.

i'd suggest fix final results, tofixed() returns string.

html select:

<select id="dec">     <option value="1">1 decimal</option>     <option value="2">2 decimals</option>     <option value="3">3 decimals</option> </select> 

in calcul():

var decs = +(document.getelementbyid('dec').value); // add line document.getelementbyid("netincome").innerhtml = (taxinc - ((taxr / 100) * taxinc)).tofixed(decs); document.getelementbyid("atre").innerhtml = (netinc - divid).tofixed(decs); 

a live demo @ jsfiddle (updated).

edit

looks i've partially missunderstood question. please check updated fiddle, can change amount of decimals @ time picking value select.