decimalToFraction.js

var Fraction = {
        convert: function( value, opts ) {
            var frac;
            if( value === 0 ) {
                frac = [ 0, 1];
            }
            else {
                if( value < 1e-6 || value > 1e20) {
                    var qc = this.quickConversion( Number( value ) );
                    if( qc[1] <= 1e20 ) {
                        var abs = Math.abs( value );
                        var sign = value/abs;
                        frac = this.fullConversion( abs.toFixed( (qc[1]+'').length-1 ));
                        frac[0] = frac[0]*sign;
                    }
                    else {
                        frac = qc;
                    }
                }
                else {
                    frac = this.fullConversion( value );
                }
            }
            return frac;
        },
        // If the fraction is small or too large this gets called instead of 
        // fullConversion method
        quickConversion: function( dec ) {
            var x = (dec.toExponential()+'').split('e');
            var d = x[0].split('.')[1];// get the number of places after the decimal
            var l = d ? d.length : 0; // maybe the coefficient is an integer;
            return [Math.pow(10,l)*x[0], Math.pow(10, Math.abs(x[1])+l)];
        },
        
        fullConversion: function( dec ) {
            //function returns a good approximation of a fraction
            //http://mathforum.org/library/drmath/view/61772.html
            //Decimal To Fraction Conversion - A Simpler Version
            //Dr Peterson
            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 [num, den];
        }
    };
    
    /*
    Usage: 
    var f = Fraction.convert(0.625);
    Output: [5, 8]
    */