Tiny optimization.

src/cover.js

@@ -1,14 +1,10 @@
 export default function(x, y) {
   if (isNaN(x = +x) || isNaN(y = +y)) return this; // ignore invalid points
 
-  var node = this._root,
-      parent,
-      i,
-      x0 = this._x0,
+  var x0 = this._x0,
       y0 = this._y0,
       x1 = this._x1,
-      y1 = this._y1,
-      z = x1 - x0;
+      y1 = this._y1;
 
   // If the quadtree has no extent, initialize them.
   // Integer extent are necessary so that if we later double the extent,
@@ -20,6 +16,11 @@ export default function(x, y) {
 
   // Otherwise, double repeatedly to cover.
   else if (x0 > x || x > x1 || y0 > y || y > y1) {
+    var z = x1 - x0,
+        node = this._root,
+        parent,
+        i;
+
     switch (i = (y < (y0 + y1) / 2) << 1 | (x < (x0 + x1) / 2)) {
       case 0: {
         do parent = new Array(4), parent[i] = node, node = parent;
@@ -46,6 +47,9 @@ export default function(x, y) {
     if (this._root && this._root.length) this._root = node;
   }
 
+  // If the quadtree covers the point already, just return.
+  else return this;
+
   this._x0 = x0;
   this._y0 = y0;
   this._x1 = x1;