I have found several solutions for crazy polygon detecting algorithms, however, my problem is a bit simpler. I have a line created by an array of indexed points. Each click on map creates a new point and appends it to the end of the array, and creates a new line from the previous end to the new one.

I would like to detect when you create a point in a way that the line folds on itself and creates a polygon. I currently create a polygon when you click the first point again. Then all the points inside get selected.

Detect this and select all points inside

Here the 4th point was created and the line closed creating a triangle. Is there a simple solution to detect this?

Edit: Using leaflet library

  • What software are you using? Make sure to include that in your question. Commented Jul 14, 2017 at 15:24
  • Question was tagged, but I added it to the Q itself, thanks.
    – Poody
    Commented Jul 17, 2017 at 5:11
  • I've never used it, so I'm not sure whether it's entirely suitable here - but it sounds like the type of problem that Turf.js was designed to solve. See turfjs.org Commented Jul 17, 2017 at 6:49

1 Answer 1


I used a modified version of an algorithm I found online. It takes line A-B, C-D and returns their intersection, if there is any that lies on both input lines. So I iterated over my point array and then if any of them created a line that intersected the last line, a polygon was created, as previously when clicked the first point.

Said algorithm

function getLineIntersection(ax, ay, bx, by, cx, cy, dx, dy) {
    var d, a, b, n1, n2, result = {
        x: null,
        y: null,
    d = ((dy - cy) * (bx - ax)) - ((dx - cx) * (by - ay));
    if (d != 0) {
        a = ay - cy;
        b = ax - cx;
        n1 = ((dx - cx) * a) - ((dy - cy) * b);
        n2 = ((bx - ax) * a) - ((by - ay) * b);
        a = n1 / d;
        b = n2 / d;
        if (a > 0 && a < 1 && b > 0 && b < 1) {
            result.x = ax + (a * (bx - ax));
            result.y = ay + (a * (by - ay));

    return result;

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