Combine two RGBA colors with JavaScript

combine-rgba-colors.js

function blendColors() {
    /* https://gist.github.com/JordanDelcros/518396da1c13f75ee057 */
    var args = [ ...arguments ];
    var base = [ 0, 0, 0, 0 ];
    var mix;
    var added;
    while (added = args.shift()) {
        if (typeof added[3] === 'undefined') {
            added[3] = 1;
        }
        if (base[3] && added[3]) { // check if both alpha channels exist.
            mix = [ 0, 0, 0, 0 ];
            mix[3] = 1 - (1 - added[3]) * (1 - base[3]); // alpha
            mix[0] = Math.round((added[0] * added[3] / mix[3]) + (base[0] * base[3] * (1 - added[3]) / mix[3])); // red
            mix[1] = Math.round((added[1] * added[3] / mix[3]) + (base[1] * base[3] * (1 - added[3]) / mix[3])); // green
            mix[2] = Math.round((added[2] * added[3] / mix[3]) + (base[2] * base[3] * (1 - added[3]) / mix[3])); // blue
        } else if (added) {
            mix = added;
        } else {
            mix = base;
        }
        base = mix;
    }
    return mix;
}

/* blendColors([ 69, 109, 160, 1 ], [ 61, 47, 82, 0.8 ]); // will return [ 63, 59, 98, 1 ] */