node_modules/popper.js/src/modifiers/inner.js

import getClientRect from '../utils/getClientRect';
import getOppositePlacement from '../utils/getOppositePlacement';

/**
 * @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 inner(data) {
  const placement = data.placement;
  const basePlacement = placement.split('-')[0];
  const { popper, reference } = data.offsets;
  const isHoriz = ['left', 'right'].indexOf(basePlacement) !== -1;

  const subtractLength = ['top', 'left'].indexOf(basePlacement) === -1;

  popper[isHoriz ? 'left' : 'top'] =
    reference[basePlacement] -
    (subtractLength ? popper[isHoriz ? 'width' : 'height'] : 0);

  data.placement = getOppositePlacement(placement);
  data.offsets.popper = getClientRect(popper);

  return data;
}