Don't add undefined values to objects when parsing JSON (#2127)

## Summary:
Previously, when parsing typed objects from JSON, we would add
`[key]: undefined` for any fields that were specified in the parser schema but
were absent from the JSON. This was a difference from the behavior of
`JSON.parse()` that could cause bugs in some cases (e.g. if a parsed
object was spread into another object, it could overwrite existing
values with `undefined` when that was not intended).

Thus, to better match the behavior of `JSON.parse()`, we now omit
`undefined` values from the parsed object if they are not present on the
original object.

This PR was inspired by test failures on Khan/webapp#28551.
I'm making this change out of an abundance of caution.

Issue: LEMS-2774

Test plan:

`yarn test`

Author: benchristel

Reviewers: jeremywiebe, handeyeco

Required Reviewers:

Approved By: jeremywiebe

Checks: ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x)

Pull Request URL: #2127

Author: benchristel

.changeset/happy-pets-breathe.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus": minor
+---
+
+Avoid adding undefined values to objects parsed from Perseus JSON when properties are missing.

packages/perseus/src/util/parse-perseus-json/general-purpose-parsers/object.test.ts

@@ -1,9 +1,10 @@
-import {assertFailure, success} from "../result";
+import {assertFailure, assertSuccess, success} from "../result";
 
 import {array} from "./array";
 import {defaulted} from "./defaulted";
 import {number} from "./number";
 import {object} from "./object";
+import {optional} from "./optional";
 import {string} from "./string";
 import {anyFailure, ctx, parseFailureWith} from "./test-helpers";
 
@@ -93,4 +94,19 @@ describe("object", () => {
 
         expect(Train({}, ctx())).toEqual(success({boxcars: []}));
     });
+
+    it("does not include fields not present on the original object", () => {
+        const Penguin = object({hat: optional(string)});
+        const result = Penguin({}, ctx());
+        assertSuccess(result);
+        expect(result.value).not.toHaveProperty("hat");
+    });
+
+    it("includes `undefined` fields from the original object", () => {
+        const Penguin = object({hat: optional(string)});
+        const result = Penguin({hat: undefined}, ctx());
+        assertSuccess(result);
+        expect(result.value).toHaveProperty("hat");
+        expect(result.value.hat).toBe(undefined);
+    });
 });

packages/perseus/src/util/parse-perseus-json/general-purpose-parsers/object.ts

@@ -19,7 +19,9 @@ export function object<S extends ObjectSchema>(
         for (const [prop, propParser] of Object.entries(schema)) {
             const result = propParser(rawValue[prop], ctx.forSubtree(prop));
             if (isSuccess(result)) {
-                ret[prop] = result.value;
+                if (result.value !== undefined || prop in rawValue) {
+                    ret[prop] = result.value;
+                }
             } else {
                 mismatches.push(...result.detail);
             }