Create mapToArray util (#1742)

This is a fairly simple helper function that allows transforming a Map into an Array using a provided transformer function. It's really an alias for `Array.from(map.entries()).map(f)`, which is nice because that invocation is somewhat tedious to write.

Test plan: The method is very simple; we've added some correspondingly simple test. Also, grepping for `Array\.from(.*\.entries())\.map` should return no hits (i.e. we've converted existing usage).

Paired with @decentralion

src/graphql/mirror.js

@@ -778,7 +778,8 @@ export class Mirror {
           ])
         );
       }),
-      Array.from(connectionsByObject.entries()).map(
+      MapUtil.mapToArray(
+        connectionsByObject,
         ([id, {typename, connections}], i) => {
           const name = `${_FIELD_PREFIXES.NODE_CONNECTIONS}${i}`;
           return b.alias(

src/util/map.js

@@ -161,3 +161,14 @@ export function pushValue<K, V>(map: Map<K, V[]>, key: K, value: V): V[] {
   arr.push(value);
   return arr;
 }
+
+/**
+ * Given a Map, transform its entries into an Array using a
+ * provided transformer function.
+ */
+export function mapToArray<K, V, R>(
+  map: Map<K, V>,
+  fn: (pair: [K, V], index: number) => R
+): R[] {
+  return Array.from(map.entries()).map(fn);
+}

src/util/map.test.js

@@ -1,6 +1,7 @@
 // @flow
 
 import * as MapUtil from "./map";
+import {mapToArray} from "./map";
 
 describe("util/map", () => {
   describe("toObject", () => {
@@ -327,4 +328,43 @@ describe("util/map", () => {
       expect(result).toBe(arr);
     });
   });
+
+  describe("mapToArray", () => {
+    const fn = ([key, val]) => ({key, val});
+
+    it("works for an empty map", () => {
+      const map = new Map();
+
+      expect(mapToArray(map, fn)).toEqual([]);
+    });
+
+    it("works for simple use case", () => {
+      const map = new Map([
+        ["foo", 1],
+        ["bar", 2],
+      ]);
+
+      const expected = [
+        {
+          key: "foo",
+          val: 1,
+        },
+        {
+          key: "bar",
+          val: 2,
+        },
+      ];
+      expect(mapToArray(map, fn)).toEqual(expected);
+    });
+
+    it("should provide the index to the function", () => {
+      const map = new Map([
+        ["foo", 1],
+        ["bar", 99],
+      ]);
+
+      const expected = [0, 1];
+      expect(mapToArray(map, (_, i) => i)).toEqual(expected);
+    });
+  });
 });