Add is.directInstanceOf() (#38)

readme.md

@@ -136,6 +136,20 @@ is.boundFunction(function () {});
 
 #### Miscellaneous
 
+##### .directInstanceOf(value, class)
+
+Returns `true` if `value` is a direct instance of `class`.
+
+```js
+is.directInstanceOf(new Error(), Error);
+//=> true
+
+class UnicornError extends Error {};
+
+is.directInstanceOf(new UnicornError(), Error);
+//=> false
+```
+
 ##### .truthy(value)
 
 Returns `true` for all values that evaluate to true in a boolean context:

source/index.ts

@@ -162,6 +162,10 @@ namespace is { // tslint:disable-line:no-namespace
 	export const arrayBuffer = isObjectOfType(TypeName.ArrayBuffer);
 	export const sharedArrayBuffer = isObjectOfType(TypeName.SharedArrayBuffer);
 
+	// TODO: Remove `object` checks when targeting ES2015 or higher
+	// See `Notes`: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/getPrototypeOf
+	export const directInstanceOf = (instance: any, klass: any) => object(instance) && object(klass) && Object.getPrototypeOf(instance) === klass.prototype;
+
 	export const truthy = (value: any) => Boolean(value);
 	export const falsy = (value: any) => !value;
 

source/tests/test.ts

@@ -507,6 +507,17 @@ test('is.dataView', t => {
 	testType(t, 'arrayBuffer');
 });
 
+test('is.directInstanceOf', t => {
+	const error = new Error();
+	const errorSubclass = new ErrorSubclassFixture();
+
+	t.true(m.directInstanceOf(error, Error));
+	t.true(m.directInstanceOf(errorSubclass, ErrorSubclassFixture));
+
+	t.false(m.directInstanceOf(error, ErrorSubclassFixture));
+	t.false(m.directInstanceOf(errorSubclass, Error));
+});
+
 test('is.truthy', t => {
 	t.true(m.truthy('unicorn'));
 	t.true(m.truthy('🦄'));