Delete batched callback before calling (#202)

If a `callback` within `batchedCallbacks` calls `assign` we have an
infinite loop. Removing the callback from `batchedCallbacks` before
executing it breaks this loop.

Consider a callback calling `assign`. `assign` will call `batch`, which
will loop through all `batchedCallbacks` and execute each one. If one of
those callbacks calls `assign` we will continue indefinitely.

By first removing the callback from `batchedCallbacks`, then executing
we ensure that when the callback calls `assign` the callback is no
longer part of the enqueued `batchedCallbacks`.

Author: joshestein

src/observable.ts

@@ -37,8 +37,10 @@ export function batch(callback: () => void) {
   callback();
   batchDepth--;
   if (batchDepth === 0) {
-    for (const [callback, state] of batchedCallbacks.entries()) callback(state);
-    batchedCallbacks.clear();
+    for (const [callback, state] of batchedCallbacks.entries()) {
+      batchedCallbacks.delete(callback);
+      callback(state);
+    }
   }
 }