Convert a decimal number to a fraction / rational number

You can use Erik Garrison's fraction.js library to do that and more fractional operations.

var f = new Fraction(2, 10000);console.log(f.numerator + '/' + f.denominator);

To to do .003 you can just do

var f = new Fraction(.003);console.log(f.numerator + '/' + f.denominator);

A little googling with the term "decimal to fraction js" the first yielded this:

http://wildreason.com/wildreason-blog/2010/javascript-convert-a-decimal-into-a-simplified-fraction/

It seems to work:

http://jsfiddle.net/VKfHH/

function HCF(u, v) {     var U = u, V = v    while (true) {        if (!(U%=V)) return V        if (!(V%=U)) return U     } }//convert a decimal into a fractionfunction fraction(decimal){    if(!decimal){        decimal=this;    }    whole = String(decimal).split('.')[0];    decimal = parseFloat("."+String(decimal).split('.')[1]);    num = "1";    for(z=0; z<String(decimal).length-2; z++){        num += "0";    }    decimal = decimal*num;    num = parseInt(num);    for(z=2; z<decimal+1; z++){        if(decimal%z==0 && num%z==0){            decimal = decimal/z;            num = num/z;            z=2;        }    }    //if format of fraction is xx/xxx    if (decimal.toString().length == 2 &&             num.toString().length == 3) {                //reduce by removing trailing 0's        decimal = Math.round(Math.round(decimal)/10);        num = Math.round(Math.round(num)/10);    }    //if format of fraction is xx/xx    else if (decimal.toString().length == 2 &&             num.toString().length == 2) {        decimal = Math.round(decimal/10);        num = Math.round(num/10);    }    //get highest common factor to simplify    var t = HCF(decimal, num);    //return the fraction after simplifying it    return ((whole==0)?"" : whole+" ")+decimal/t+"/"+num/t;}// Test italert(fraction(0.0002)); // "1/5000"

I used this site http://mathforum.org/library/drmath/view/51886.html to build a function but as the article mentions you will get an unreasonable large number for radicals or pi.

Hope it helps though.

function Fraction(){}Fraction.prototype.convert = function(x, improper){    improper = improper || false;    var abs = Math.abs(x);    this.sign = x/abs;    x = abs;    var stack = 0;    this.whole = !improper ? Math.floor(x) : 0;    var fractional = !improper ? x-this.whole : abs;    /*recursive function that transforms the fraction*/    function recurs(x){        stack++;        var intgr = Math.floor(x); //get the integer part of the number        var dec = (x - intgr); //get the decimal part of the number        if(dec < 0.0019 || stack > 20) return [intgr,1]; //return the last integer you divided by        var num = recurs(1/dec); //call the function again with the inverted decimal part        return[intgr*num[0]+num[1],num[0]]    }    var t = recurs(fractional);     this.numerator = t[0];    this.denominator = t[1];}Fraction.prototype.toString = function(){    var l  = this.sign.toString().length;    var sign = l === 2 ? '-' : '';    var whole = this.whole !== 0 ? this.sign*this.whole+' ': sign;    return whole+this.numerator+'/'+this.denominator;}//var frac = new Fraction()//frac.convert(2.56, false)//console.log(frac.toString())//use frac.convert(2.56,true) to get it as an improper fraction

If you just want a self-contained function that only returns the numerator and denominator then use the function below.

var toFraction = function (dec) {    var is_neg = dec < 0;    dec = Math.abs(dec);    var done = false;    //you can adjust the epsilon to a larger number if you don't need very high precision    var n1 = 0, d1 = 1, n2 = 1, d2 = 0, n = 0, q = dec, epsilon = 1e-13;    while (!done) {        n++;        if (n > 10000) {            done = true;        }        var a = parseInt(q);        var num = n1 + a * n2;        var den = d1 + a * d2;        var e = (q - a);        if (e < epsilon) {            done = true;        }        q = 1 / e;        n1 = n2;        d1 = d2;        n2 = num;        d2 = den;        if (Math.abs(num / den - dec) < epsilon || n > 30) {            done = true;        }    }    return [is_neg ? -num : num, den];};//Usage://var frac = toFraction(0.5);//console.log(frac)//Output: [ 1, 2 ]