Perf: memoize collectSubfields

src/execution/execute.js

@@ -12,6 +12,7 @@ import { GraphQLError, locatedError } from '../error';
 import invariant from '../jsutils/invariant';
 import isInvalid from '../jsutils/isInvalid';
 import isNullish from '../jsutils/isNullish';
+import memoize3 from '../jsutils/memoize3';
 import type { ObjMap } from '../jsutils/ObjMap';
 import type { MaybePromise } from '../jsutils/MaybePromise';
 
@@ -1236,6 +1237,21 @@ function collectAndExecuteSubfields(
   result: mixed,
 ): mixed {
   // Collect sub-fields to execute to complete this value.
+  const subFieldNodes = collectSubfields(exeContext, returnType, fieldNodes);
+  return executeFields(exeContext, returnType, result, path, subFieldNodes);
+}
+
+/**
+ * A memoized collection of relevant subfields in the context of the return
+ * type. Memoizing ensures the subfields are not repeatedly calculated, which
+ * saves overhead when resolving lists of values.
+ */
+const collectSubfields = memoize3(_collectSubfields);
+function _collectSubfields(
+  exeContext: ExecutionContext,
+  returnType: GraphQLObjectType,
+  fieldNodes: $ReadOnlyArray<FieldNode>,
+): ObjMap<Array<FieldNode>> {
   let subFieldNodes = Object.create(null);
   const visitedFragmentNames = Object.create(null);
   for (let i = 0; i < fieldNodes.length; i++) {
@@ -1250,8 +1266,7 @@ function collectAndExecuteSubfields(
       );
     }
   }
-
-  return executeFields(exeContext, returnType, result, path, subFieldNodes);
+  return subFieldNodes;
 }
 
 /**

src/jsutils/memoize3.js

@@ -0,0 +1,44 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+/**
+ * Memoizes the provided three-argument function.
+ */
+export default function memoize3<T: (a1: any, a2: any, a3: any) => any>(
+  fn: T,
+): T {
+  let cache0;
+  function memoized(a1, a2, a3) {
+    if (!cache0) {
+      cache0 = new WeakMap();
+    }
+    let cache1 = cache0.get(a1);
+    let cache2;
+    if (cache1) {
+      cache2 = cache1.get(a2);
+      if (cache2) {
+        const cachedValue = cache2.get(a3);
+        if (cachedValue) {
+          return cachedValue;
+        }
+      }
+    } else {
+      cache1 = new WeakMap();
+      cache0.set(a1, cache1);
+    }
+    if (!cache2) {
+      cache2 = new WeakMap();
+      cache1.set(a2, cache2);
+    }
+    const newValue = fn.apply(this, arguments);
+    cache2.set(a3, newValue);
+    return newValue;
+  }
+  return (memoized: any);
+}