pnpoly-with-clipped-polygons.js

function isInsideClippedPolygon(polygon, mouseX, mouseY) {    
    let c = false;
        
    // make sure there are at least 3 visible nodes
    var countv = 0;
    for (let k=0; k < polygon.length && countv < 3; k++) {
        if (polygon[k].visible == true)
            countv++;
    }
    if (countv < 3)
        return false;
    // get the first visible node
    let firstv = null;
    for (let k=0; k < polygon.length; k++) {
        if (polygon[k].visible == true) {
            firstv = k;
            break;
        }
    }
    
    // raycast most-recent-visible-node -> next-visible-node
    const mrv = firstv;
    for (let i=mrv+1; i < polygon.length; i++) {
        if (polygon[i].visible == false)
            continue;    
        let j = mrv;
        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;
        }            
        mrv = i;
    }
    
    // raycast most-recent-visible-node -> first-visible-node
    const ix = polygon[firstv].x;
    const iy = polygon[firstv].y;
    const jx = polygon[mrv].x;
    const jy = polygon[mrv].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;
}

The variable firstv, c would have to be let instead of const to be reassignable.

Corrected firstv and c per @codeSourc3