node_modules/popper.js/src/modifiers/arrow.js

import getClientRect from '../utils/getClientRect';
import getOuterSizes from '../utils/getOuterSizes';
import isModifierRequired from '../utils/isModifierRequired';
import getStyleComputedProperty from '../utils/getStyleComputedProperty';

/**
 * @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 arrow(data, options) {
  // arrow depends on keepTogether in order to work
  if (!isModifierRequired(data.instance.modifiers, 'arrow', 'keepTogether')) {
    return data;
  }

  let arrowElement = options.element;

  // if arrowElement is a string, suppose it's a CSS selector
  if (typeof arrowElement === 'string') {
    arrowElement = data.instance.popper.querySelector(arrowElement);

    // if arrowElement is not found, don't run the modifier
    if (!arrowElement) {
      return data;
    }
  } else {
    // if the arrowElement isn't a query selector we must check that the
    // provided DOM node is child of its popper node
    if (!data.instance.popper.contains(arrowElement)) {
      console.warn(
        'WARNING: `arrow.element` must be child of its popper element!'
      );
      return data;
    }
  }

  const placement = data.placement.split('-')[0];
  const { popper, reference } = data.offsets;
  const isVertical = ['left', 'right'].indexOf(placement) !== -1;

  const len = isVertical ? 'height' : 'width';
  const sideCapitalized = isVertical ? 'Top' : 'Left';
  const side = sideCapitalized.toLowerCase();
  const altSide = isVertical ? 'left' : 'top';
  const opSide = isVertical ? 'bottom' : 'right';
  const arrowElementSize = getOuterSizes(arrowElement)[len];

  //
  // extends keepTogether behavior making sure the popper and its
  // reference have enough pixels in conjunction
  //

  // top/left side
  if (reference[opSide] - arrowElementSize < popper[side]) {
    data.offsets.popper[side] -=
      popper[side] - (reference[opSide] - arrowElementSize);
  }
  // bottom/right side
  if (reference[side] + arrowElementSize > popper[opSide]) {
    data.offsets.popper[side] +=
      reference[side] + arrowElementSize - popper[opSide];
  }
  data.offsets.popper = getClientRect(data.offsets.popper);

  // compute center of the popper
  const center = reference[side] + reference[len] / 2 - arrowElementSize / 2;

  // Compute the sideValue using the updated popper offsets
  // take popper margin in account because we don't have this info available
  const css = getStyleComputedProperty(data.instance.popper);
  const popperMarginSide = parseFloat(css[`margin${sideCapitalized}`]);
  const popperBorderSide = parseFloat(css[`border${sideCapitalized}Width`]);
  let sideValue =
    center - data.offsets.popper[side] - popperMarginSide - popperBorderSide;

  // prevent arrowElement from being placed not contiguously to its popper
  sideValue = Math.max(Math.min(popper[len] - arrowElementSize, sideValue), 0);

  data.arrowElement = arrowElement;
  data.offsets.arrow = {
    [side]: Math.round(sideValue),
    [altSide]: '', // make sure to unset any eventual altSide value from the DOM node
  };

  return data;
}