From 2c7d07162b9a7bd0186e045b678272e53bd991a7 Mon Sep 17 00:00:00 2001
From: JSap0914 <JSap0914@users.noreply.github.com>
Date: Sat, 27 Jun 2026 18:46:40 +0900
Subject: [PATCH] fix(rhumb-bearing): return NaN for coincident points

When start and end points are identical, rhumbBearing() was returning 0
(north) because Math.atan2(0, 0) === 0. This is misleading since no
bearing can be defined for a zero-length segment.

The Geodesy library (which this implementation is adapted from) explicitly
guards for this case and returns NaN. Align turf's behaviour accordingly.

Closes #2478
---
 packages/turf-rhumb-bearing/index.ts |  4 ++++
 packages/turf-rhumb-bearing/test.ts  | 12 ++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/packages/turf-rhumb-bearing/index.ts b/packages/turf-rhumb-bearing/index.ts
index c3d48ccc9f..f821a31d11 100644
--- a/packages/turf-rhumb-bearing/index.ts
+++ b/packages/turf-rhumb-bearing/index.ts
@@ -54,6 +54,10 @@ function rhumbBearing(
  * var d = p1.rhumbBearingTo(p2); // 116.7 m
  */
 function calculateRhumbBearing(from: number[], to: number[]) {
+  // Coincident points have no defined bearing (matches Geodesy reference impl)
+  if (from[0] === to[0] && from[1] === to[1]) {
+    return NaN;
+  }
   // φ => phi
   // Δλ => deltaLambda
   // Δψ => deltaPsi
diff --git a/packages/turf-rhumb-bearing/test.ts b/packages/turf-rhumb-bearing/test.ts
index 377a0c9a41..a9f40767fc 100644
--- a/packages/turf-rhumb-bearing/test.ts
+++ b/packages/turf-rhumb-bearing/test.ts
@@ -46,6 +46,18 @@ test("bearing", (t) => {
     );
   });
 
+  // Coincident points have no defined bearing — must return NaN, not 0
+  // Regression test for https://github.com/Turfjs/turf/issues/2478
+  const coincident = point([5, 5]);
+  t.ok(
+    isNaN(rhumbBearing(coincident, coincident)),
+    "coincident points return NaN"
+  );
+  t.ok(
+    isNaN(rhumbBearing(coincident, coincident, { final: true })),
+    "coincident points return NaN (final bearing)"
+  );
+
   t.throws(() => {
     rhumbBearing(point([12, -54]), "point");
   }, "invalid point");