js/walk: allow callback to return boolean in addition to Promise (#1511)

js/src/walk-test.js

@@ -132,14 +132,26 @@ suite('walk', () => {
     assert.equal(0, expected.size);
   });
 
-  test('cb-default', async () => {
-    const rv = ds.writeValue(42);
-    const expected = new Set([rv, 42]);
-    await walk(rv, ds, async v => {
-      assert.isOk(expected.delete(v));
-      // return nothing -- default should be to recurse.
-    });
-    assert.equal(0, expected.size);
+  test('cb-should-recurse', async () => {
+    const testShouldRecurse = async (cb, expectRecurse) => {
+      const rv = ds.writeValue(42);
+      const expected = new Set([rv, 42]);
+      await walk(rv, ds, v => {
+        assert.isOk(expected.delete(v));
+        return cb();
+      });
+      assert.equal(expectRecurse ? 0 : 1, expected.size);
+    };
+
+    // Return void, Promise<void>, true, or Promise<true> -- should recurse.
+    await testShouldRecurse(() => { return; }, true); // eslint-disable-line
+    await testShouldRecurse(() => Promise.resolve(), true);
+    await testShouldRecurse(() => true, true);
+    await testShouldRecurse(() => Promise.resolve(true), true);
+
+    // Return true or Promise<true> -- should stop
+    await testShouldRecurse(() => false, false);
+    await testShouldRecurse(() => Promise.resolve(false), false);
   });
 });
 

js/src/walk.js

@@ -10,16 +10,20 @@ import Struct, {StructMirror} from './struct.js';
 import type Database from './database.js';
 import type {valueOrPrimitive} from './value.js';
 
+type walkCb = (v: valueOrPrimitive) => ?bool | Promise<?bool>;
+
 // Invokes |cb| once for |v| and each of its descendants. The returned promise is resolved when all
 // invocations to |cb| have been resolved.
 //
-// The return value of |cb| indicates whether to recurse further into the tree. Return false to
-// skip a node's children.
+// The return value of |cb| indicates whether to recurse further into the tree. Return |false| or a
+// Promise which resolves to |false| to skip a node's children.
 //
-// For convenience, if |cb| returns |undefined|, the default is |true|.
-export default async function walk(v: valueOrPrimitive, ds: Database,
-                                   cb: (v: valueOrPrimitive) => Promise<?bool>): Promise<void> {
-  let cont = await cb(v);
+// For convenience, if |cb| returns |undefined| or a Promise to |undefined|, the default is |true|.
+export default async function walk(v: valueOrPrimitive, ds: Database, cb: walkCb): Promise<void> {
+  let cont = cb(v);
+  if (cont instanceof Promise) {
+    cont = await cont;
+  }
   if (cont === undefined) {
     cont = true;
   }