Here is a standalone function:

function rgb2hsv (r, g, b) {

    let rabs, gabs, babs, rr, gg, bb, h, s, v, diff, diffc, percentRoundFn;

    rabs = r / 255;

    gabs = g / 255;

    babs = b / 255;

    v = Math.max(rabs, gabs, babs),

    diff = v - Math.min(rabs, gabs, babs);

    diffc = c => (v - c) / 6 / diff + 1 / 2;

    percentRoundFn = num => Math.round(num * 100) / 100;

    if (diff == 0) {

        h = s = 0;

    } else {

        s = diff / v;

        rr = diffc(rabs);

        gg = diffc(gabs);

        bb = diffc(babs);



        if (rabs === v) {

            h = bb - gg;

        } else if (gabs === v) {

            h = (1 / 3) + rr - bb;

        } else if (babs === v) {

            h = (2 / 3) + gg - rr;

        }

        if (h < 0) {

            h += 1;

        }else if (h > 1) {

            h -= 1;

        }

    }

    return {

        h: Math.round(h * 360),

        s: percentRoundFn(s * 100),

        v: percentRoundFn(v * 100)

    };

}

And how to use it:

console.log( rgb2hsv(60, 120, 180) );

// {h: 210, s: 66.67, v: 70.59}