add map.getMaxBounds method

Credit to @lamuertepeluda which provided some of the basis to this
commit at #4029 (comment)

src/ui/map.js

@@ -406,6 +406,22 @@ class Map extends Camera {
         return bounds;
     }
 
+    /**
+     * Gets the map's geographical bounds.
+     *
+     * Returns the LngLatBounds by which pan and zoom operations on the map are constrained.
+     *
+     * @returns {LngLatBounds | null} The maximum bounds the map is constrained to, or `null` if none set.
+     */
+    getMaxBounds () {
+        if (!(!Array.isArray(this.transform.latRange) || this.transform.latRange.length === 0) &&
+            (!Array.isArray(this.transform.lngRange) || this.transform.lngRange.length === 0)) {
+            return null;
+        }
+        return new LngLatBounds([this.transform.lngRange[0], this.transform.latRange[0]],
+            [this.transform.lngRange[1], this.transform.latRange[1]]);
+    }
+
     /**
      * Sets or clears the map's geographical bounds.
      *
@@ -434,6 +450,7 @@ class Map extends Camera {
         return this;
 
     }
+
     /**
      * Sets or clears the map's minimum zoom level.
      * If the map's current zoom level is lower than the new minimum,

test/unit/ui/map.test.js

@@ -645,6 +645,24 @@ test('Map', (t) => {
         t.end();
     });
 
+    t.test('#getMaxBounds', (t) => {
+        t.test('returns null when no bounds set', (t) => {
+            const map = createMap({zoom:0});
+            t.equal(map.getMaxBounds(), null);
+            t.end();
+        });
+
+        t.test('returns bounds', (t) => {
+            const map = createMap({zoom:0});
+            const bounds = [[-130.4297, 50.0642], [-61.52344, 24.20688]];
+            map.setMaxBounds(bounds);
+            t.deepEqual(map.getMaxBounds().toArray(), bounds);
+            t.end();
+        });
+
+        t.end();
+    });
+
     t.test('#setMinZoom', (t) => {
         const map = createMap({zoom:5});
         map.setMinZoom(3.5);