Source: \core\math\shapes\Polygon.js

    import Point from '../Point';
import { SHAPES } from '../../const';

/**
 * @class
 * @memberof PIXI
 */
export default class Polygon
{
    /**
     * @param {PIXI.Point[]|number[]} points - This can be an array of Points
     *  that form the polygon, a flat array of numbers that will be interpreted as [x,y, x,y, ...], or
     *  the arguments passed can be all the points of the polygon e.g.
     *  `new PIXI.Polygon(new PIXI.Point(), new PIXI.Point(), ...)`, or the arguments passed can be flat
     *  x,y values e.g. `new Polygon(x,y, x,y, x,y, ...)` where `x` and `y` are Numbers.
     */
    constructor(...points)
    {
        if (Array.isArray(points[0]))
        {
            points = points[0];
        }

        // if this is an array of points, convert it to a flat array of numbers
        if (points[0] instanceof Point)
        {
            const p = [];

            for (let i = 0, il = points.length; i < il; i++)
            {
                p.push(points[i].x, points[i].y);
            }

            points = p;
        }

        this.closed = true;

        /**
         * An array of the points of this polygon
         *
         * @member {number[]}
         */
        this.points = points;

        /**
         * The type of the object, mainly used to avoid `instanceof` checks
         *
         * @member {number}
         * @readOnly
         * @default PIXI.SHAPES.POLY
         * @see PIXI.SHAPES
         */
        this.type = SHAPES.POLY;
    }

    /**
     * Creates a clone of this polygon
     *
     * @return {PIXI.Polygon} a copy of the polygon
     */
    clone()
    {
        return new Polygon(this.points.slice());
    }

    /**
     * Closes the polygon, adding points if necessary.
     *
     */
    close()
    {
        const points = this.points;

        // close the poly if the value is true!
        if (points[0] !== points[points.length - 2] || points[1] !== points[points.length - 1])
        {
            points.push(points[0], points[1]);
        }
    }

    /**
     * Checks whether the x and y coordinates passed to this function are contained within this polygon
     *
     * @param {number} x - The X coordinate of the point to test
     * @param {number} y - The Y coordinate of the point to test
     * @return {boolean} Whether the x/y coordinates are within this polygon
     */
    contains(x, y)
    {
        let inside = false;

        // use some raycasting to test hits
        // https://github.com/substack/point-in-polygon/blob/master/index.js
        const length = this.points.length / 2;

        for (let i = 0, j = length - 1; i < length; j = i++)
        {
            const xi = this.points[i * 2];
            const yi = this.points[(i * 2) + 1];
            const xj = this.points[j * 2];
            const yj = this.points[(j * 2) + 1];
            const intersect = ((yi > y) !== (yj > y)) && (x < ((xj - xi) * ((y - yi) / (yj - yi))) + xi);

            if (intersect)
            {
                inside = !inside;
            }
        }

        return inside;
    }
}