Add enum parser utility

ts/environment.ts

@@ -1,6 +1,8 @@
 // Copyright 2021 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
+import { makeEnumParser } from './util/enum';
+
 // Many places rely on this enum being a string.
 export enum Environment {
   Development = 'development',
@@ -33,17 +35,7 @@ export function setEnvironment(env: Environment): void {
   environment = env;
 }
 
-const ENVIRONMENTS_BY_STRING = new Map<string, Environment>([
-  ['development', Environment.Development],
-  ['production', Environment.Production],
-  ['staging', Environment.Staging],
-  ['test', Environment.Test],
-  ['test-lib', Environment.TestLib],
-]);
-export function parseEnvironment(value: unknown): Environment {
-  if (typeof value !== 'string') {
-    return Environment.Production;
-  }
-  const result = ENVIRONMENTS_BY_STRING.get(value);
-  return result || Environment.Production;
-}
+export const parseEnvironment = makeEnumParser(
+  Environment,
+  Environment.Production
+);

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);
+    });
+  });
+});

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);
+}