Body Fat Calculator

3. Javascript

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The final step is to install the Javascript that does the actual calculations for the body fat calculator. All you need to do is to copy the following code and save it as fatcalc.js.

// Body Fat Calculator
// copyright 23rd April 2006, by Stephen Chapman
// permission to use this Javascript on your web page is granted
// provided that all of the code in this script (including these
// comments) is used without any alteration
function valButton(btn) {var cnt = -1;for (var i=btn.length-1; i > -1; i--) {   if (btn[i].checked) {cnt = i; i = -1;}}if (cnt > -1) return btn[cnt].value;else return null;}function stripBlanks(fld) {var result = "";var c = 0;for (i=0; i < fld.length; i++) {if (fld.charAt(i) != " " || c > 0) {result += fld.charAt(i);if (fld.charAt(i) != " ") c = result.length;}}return result.substr(0,c);}function maleFat(h,n,w) {return thou(495/(1.0324-0.19077*(Math.log(w-n)/Math.LN10)+0.15456*(Math.log(h)/Math.LN10))-450);}function femaleFat(h,n,w,r) {return thou(495/(1.29579-0.35004*(Math.log(w+r-n)/Math.LN10)+0.22100*(Math.log(h)/Math.LN10))-450);}function thou(n) {return Math.round(n*10)/10+'%';}function sex(thisform) {var s = valButton(thisform.s);thisform.r.disabled = (s != 'f');}function calc(thisform) {var s = valButton(thisform.s);if (s == null) {alert("You must enter your sex");return false;}var d = valButton(thisform.d);if (d == null) {alert("You must choose centimetres or inches");return false;}d = Number(d);var h = stripBlanks(thisform.h.value);if (h == '') {alert("You must enter your height");thisform.h.focus();return false;}if (h != Number(h) || (h = Number(h*d)) < 100 || h > 250) {alert("Invalid height entered");thisform.h.focus();return false;}var n = stripBlanks(thisform.n.value);if (n == '') {alert("You must enter your neck size");thisform.n.focus();return false;}if (n != Number(n) || (n = Number(n*d)) < 20 || n > 60) {alert("Invalid neck size entered");thisform.n.focus();return false;}var w = stripBlanks(thisform.w.value);if (w == '') {alert("You must enter your waist size");thisform.w.focus();return false;}if (w != Number(w) || (w = Number(w*d)) < 30 || w > h) {alert("Invalid waist size entered");thisform.w.focus();return false;}var r = 0;if (s == 'f') {r = stripBlanks(thisform.r.value);if (r == '') {alert("You must enter your hip size");thisform.r.focus();return false;}if (r != Number(r) || (r = Number(r*d)) < 40 || h > 200) {alert("Invalid hip size entered");thisform.r.focus();return false;}}switch(s) {case 'm': thisform.f.value = maleFat(h,n,w); break; case 'f': thisform.f.value = femaleFat(h,n,w,r);}}

Most of that code is needed to validate the values that your visitor enters into the form in order to make sure that the values are numeric and within a reasonable range. The actual calculations themselves (if you are interested in what those formula actually are) can be found in the maleFat() and femaleFat() functions.

Guide since 2004

Stephen Chapman
JavaScript Guide

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