Add enum parser utility

ts/util/enum.ts

@@ -0,0 +1,30 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+/**
+ * Turn a string (like "green") into its enum type (like `Color.Green`). Useful when
+ * deserializing strings into enum types.
+ *
+ * It only supports enums with string values. It could theoretically support more, but:
+ *
+ * 1. It's easier to debug. A serialized value of "Green" is easier to associate with
+ *    `Color.Green` than a serialized value of 2.
+ * 2. TypeScript's default uses numeric enum values. Because the stability of values is
+ *    important and it's easy to mess up the stability of values (e.g., by reordering the
+ *    enum), these are discouraged here.
+ *
+ * Again: no "hard" technical reason why this only supports strings; it's to encourage
+ * good behavior.
+ */
+export function makeEnumParser<
+  TEnumKey extends string,
+  TEnumValue extends string
+>(
+  enumToParse: Record<TEnumKey, TEnumValue>,
+  defaultValue: TEnumValue
+): (value: unknown) => TEnumValue {
+  const enumValues = new Set(Object.values(enumToParse));
+  const isEnumValue = (value: unknown): value is TEnumValue =>
+    typeof value === 'string' && enumValues.has(value);
+  return value => (isEnumValue(value) ? value : defaultValue);
+}