Add is.enumCase and assert.enumCase (#150)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

readme.md

@@ -212,6 +212,25 @@ is.boundFunction(function () {});
 ##### .sharedArrayBuffer(value)
 ##### .dataView(value)
 
+##### .enumCase(value, enum)
+
+**TypeScript-only**
+
+Returns `true` if `value` is a member of `enum`.
+
+```ts
+enum Direction {
+	Ascending = 'ascending',
+	Descending = 'descending'
+}
+
+is.enumCase('ascending', Direction);
+//=> true
+
+is.enumCase('other', Direction);
+//=> false
+```
+
 #### Emptiness
 
 ##### .emptyString(value)

source/index.ts

@@ -230,6 +230,7 @@ is.bigUint64Array = isObjectOfType<BigUint64Array>('BigUint64Array');
 is.arrayBuffer = isObjectOfType<ArrayBuffer>('ArrayBuffer');
 is.sharedArrayBuffer = isObjectOfType<SharedArrayBuffer>('SharedArrayBuffer');
 is.dataView = isObjectOfType<DataView>('DataView');
+is.enumCase = <T = unknown>(value: unknown, targetEnum: T) => Object.values(targetEnum).includes(value as string);
 
 is.directInstanceOf = <T>(instance: unknown, class_: Class<T>): instance is T => Object.getPrototypeOf(instance) === class_.prototype;
 is.urlInstance = (value: unknown): value is URL => isObjectOfType<URL>('URL')(value);
@@ -501,6 +502,7 @@ interface Assert {
 	arrayBuffer: (value: unknown) => asserts value is ArrayBuffer;
 	sharedArrayBuffer: (value: unknown) => asserts value is SharedArrayBuffer;
 	dataView: (value: unknown) => asserts value is DataView;
+	enumCase: <T = unknown>(value: unknown, targetEnum: T) => asserts value is T[keyof T];
 	urlInstance: (value: unknown) => asserts value is URL;
 	urlString: (value: unknown) => asserts value is string;
 	truthy: (value: unknown) => asserts value is unknown;
@@ -601,6 +603,7 @@ export const assert: Assert = {
 	arrayBuffer: (value: unknown): asserts value is ArrayBuffer => assertType(is.arrayBuffer(value), 'ArrayBuffer', value),
 	sharedArrayBuffer: (value: unknown): asserts value is SharedArrayBuffer => assertType(is.sharedArrayBuffer(value), 'SharedArrayBuffer', value),
 	dataView: (value: unknown): asserts value is DataView => assertType(is.dataView(value), 'DataView', value),
+	enumCase: <T = unknown>(value: unknown, targetEnum: T): asserts value is T[keyof T] => assertType(is.enumCase(value, targetEnum), 'EnumCase', value),
 	urlInstance: (value: unknown): asserts value is URL => assertType(is.urlInstance(value), 'URL', value),
 	urlString: (value: unknown): asserts value is string => assertType(is.urlString(value), AssertionTypeDescription.urlString, value),
 	truthy: (value: unknown): asserts value is unknown => assertType(is.truthy(value), AssertionTypeDescription.truthy, value),

test/test.ts

@@ -832,6 +832,23 @@ test('is.dataView', t => {
 	testType(t, 'dataView');
 });
 
+test('is.enumCase', t => {
+	enum NonNumericalEnum {
+		Key1 = 'key1',
+		Key2 = 'key2',
+	}
+
+	t.true(is.enumCase('key1', NonNumericalEnum));
+	t.notThrows(() => {
+		assert.enumCase('key1', NonNumericalEnum);
+	});
+
+	t.false(is.enumCase('invalid', NonNumericalEnum));
+	t.throws(() => {
+		assert.enumCase('invalid', NonNumericalEnum);
+	});
+});
+
 test('is.directInstanceOf', t => {
 	const error = new Error();
 	const errorSubclass = new ErrorSubclassFixture();