C-to-js ported of sass’s mix https://github.com/sass/libsass/blob/0e6b4a2850092356aa3ece07c6b249f0221caced/functions.cpp#L209

mix.cpp

Signature mix_sig = "mix($color-1, $color-2, $weight: 50%)";
BUILT_IN(mix)
{
  Color*  color1 = ARG("$color-1", Color);
  Color*  color2 = ARG("$color-2", Color);
  Number* weight = ARGR("$weight", Number, 0, 100);

  double p = weight->value()/100;
  double w = 2*p - 1;
  double a = color1->a() - color2->a();

  double w1 = (((w * a == -1) ? w : (w + a)/(1 + w*a)) + 1)/2.0;
  double w2 = 1 - w1;

  return new (ctx.mem) Color(pstate,
             std::round(w1*color1->r() + w2*color2->r()),
             std::round(w1*color1->g() + w2*color2->g()),
             std::round(w1*color1->b() + w2*color2->b()),
             color1->a()*p + color2->a()*(1-p));
}

mix.js

function mix(color1, color2, weight) {
  var weight = weight || 50;

  var p = weight / 100;
  var w = 2 * p - 1;
  var a = color1.alpha() - color2.alpha();

  var w1 = (((w * a == -1) ? w : (w + a)/(1 + w*a)) + 1) / 2.0;
  var w2 = 1 - w1;

  return this
    .rgb(
      w1 * color1.red() + w2 * color2.red(),
      w1 * color1.green() + w2 * color2.green(),
      w1 * color1.blue() + w2 * color2.blue()
    )
    .alpha(color1.alpha() * p + color2.alpha() * (1 - p));
}