Add enum parser utility

ts/test-both/util/enum_test.ts

@@ -0,0 +1,37 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { assert } from 'chai';
+
+import { makeEnumParser } from '../../util/enum';
+
+describe('enum utils', () => {
+  describe('makeEnumParser', () => {
+    enum Color {
+      Red = 'red',
+      Green = 'green',
+      Blue = 'blue',
+    }
+
+    const parse = makeEnumParser(Color, Color.Blue);
+
+    it('returns a parser that returns the default value if passed a non-string', () => {
+      [undefined, null, 0, 1, 123].forEach(serializedValue => {
+        const result: Color = parse(serializedValue);
+        assert.strictEqual(result, Color.Blue);
+      });
+    });
+
+    it('returns a parser that returns the default value if passed a string not in the enum', () => {
+      ['', 'garbage', 'RED'].forEach(serializedValue => {
+        const result: Color = parse(serializedValue);
+        assert.strictEqual(result, Color.Blue);
+      });
+    });
+
+    it('returns a parser that parses enum values', () => {
+      const result: Color = parse('green');
+      assert.strictEqual(result, Color.Green);
+    });
+  });
+});