pnpoly.js

function isInsidePolygon(polygon, mouseX, mouseY) {    
    const c = false;
    
    for (let i=1, j=0; i < polygon.length; i++, j++) {
        const ix = polygon[i].x;
        const iy = polygon[i].y;
        const jx = polygon[j].x;
        const jy = polygon[j].y;
        const iySide = (iy > mouseY);
        const jySide = (jy > mouseY);
        
        if (iySide != jySide) {
            const intersectX = (jx-ix) * (mouseY-iy) / (jy-iy) + ix;
            if (mouseX < intersectX)
                c = !c;
        }            
    }
    return c;
}