function dropshadow(a, b, v) {
    var params = [];
    var unit = 'px';

    for (var i = 0; i < 3; i += 1) {
      // eslint-disable-next-line no-bitwise
      params[i] = ((numbers(a[i], b[i], v) * 100 >> 0) / 100) + unit;
    }
    return ("drop-shadow(" + (params.concat(colors(a[3], b[3], v)).join(' ')) + ")");
  }
Sets the `drop-shadow` sub-property update function.
* disimbiguation `dropshadow` interpolation function and `dropShadow` property
@param {string} tweenProp the property name