node_modules/popper.js/src/modifiers/shift.js

/**
 * @function
 * @memberof Modifiers
 * @argument {Object} data - The data object generated by `update` method
 * @argument {Object} options - Modifiers configuration and options
 * @returns {Object} The data object, properly modified
 */
export default function shift(data) {
  const placement = data.placement;
  const basePlacement = placement.split('-')[0];
  const shiftvariation = placement.split('-')[1];

  // if shift shiftvariation is specified, run the modifier
  if (shiftvariation) {
    const { reference, popper } = data.offsets;
    const isVertical = ['bottom', 'top'].indexOf(basePlacement) !== -1;
    const side = isVertical ? 'left' : 'top';
    const measurement = isVertical ? 'width' : 'height';

    const shiftOffsets = {
      start: { [side]: reference[side] },
      end: {
        [side]: reference[side] + reference[measurement] - popper[measurement],
      },
    };

    data.offsets.popper = { ...popper, ...shiftOffsets[shiftvariation] };
  }

  return data;
}