function computeStyle()

in js/popper.js [1284:1373]
60 lines of code
14 McCabe index (conditional complexity)

function computeStyle(data, options) {
  var x = options.x,
      y = options.y;
  var popper = data.offsets.popper;

  // Remove this legacy support in Popper.js v2

  var legacyGpuAccelerationOption = find(data.instance.modifiers, function (modifier) {
    return modifier.name === 'applyStyle';
  }).gpuAcceleration;
  if (legacyGpuAccelerationOption !== undefined) {
    console.warn('WARNING: `gpuAcceleration` option moved to `computeStyle` modifier and will not be supported in future versions of Popper.js!');
  }
  var gpuAcceleration = legacyGpuAccelerationOption !== undefined ? legacyGpuAccelerationOption : options.gpuAcceleration;

  var offsetParent = getOffsetParent(data.instance.popper);
  var offsetParentRect = getBoundingClientRect(offsetParent);

  // Styles
  var styles = {
    position: popper.position
  };

  var offsets = getRoundedOffsets(data, window.devicePixelRatio < 2 || !isFirefox);

  var sideA = x === 'bottom' ? 'top' : 'bottom';
  var sideB = y === 'right' ? 'left' : 'right';

  // if gpuAcceleration is set to `true` and transform is supported,
  //  we use `translate3d` to apply the position to the popper we
  // automatically use the supported prefixed version if needed
  var prefixedProperty = getSupportedPropertyName('transform');

  // now, let's make a step back and look at this code closely (wtf?)
  // If the content of the popper grows once it's been positioned, it
  // may happen that the popper gets misplaced because of the new content
  // overflowing its reference element
  // To avoid this problem, we provide two options (x and y), which allow
  // the consumer to define the offset origin.
  // If we position a popper on top of a reference element, we can set
  // `x` to `top` to make the popper grow towards its top instead of
  // its bottom.
  var left = void 0,
      top = void 0;
  if (sideA === 'bottom') {
    // when offsetParent is <html> the positioning is relative to the bottom of the screen (excluding the scrollbar)
    // and not the bottom of the html element
    if (offsetParent.nodeName === 'HTML') {
      top = -offsetParent.clientHeight + offsets.bottom;
    } else {
      top = -offsetParentRect.height + offsets.bottom;
    }
  } else {
    top = offsets.top;
  }
  if (sideB === 'right') {
    if (offsetParent.nodeName === 'HTML') {
      left = -offsetParent.clientWidth + offsets.right;
    } else {
      left = -offsetParentRect.width + offsets.right;
    }
  } else {
    left = offsets.left;
  }
  if (gpuAcceleration && prefixedProperty) {
    styles[prefixedProperty] = 'translate3d(' + left + 'px, ' + top + 'px, 0)';
    styles[sideA] = 0;
    styles[sideB] = 0;
    styles.willChange = 'transform';
  } else {
    // othwerise, we use the standard `top`, `left`, `bottom` and `right` properties
    var invertTop = sideA === 'bottom' ? -1 : 1;
    var invertLeft = sideB === 'right' ? -1 : 1;
    styles[sideA] = top * invertTop;
    styles[sideB] = left * invertLeft;
    styles.willChange = sideA + ', ' + sideB;
  }

  // Attributes
  var attributes = {
    'x-placement': data.placement
  };

  // Update `data` attributes, styles and arrowStyles
  data.attributes = _extends({}, attributes, data.attributes);
  data.styles = _extends({}, styles, data.styles);
  data.arrowStyles = _extends({}, data.offsets.arrow, data.arrowStyles);

  return data;
}