Add is.asyncFunction (#20)

index.js

@@ -85,7 +85,10 @@ is.promise = x => is.nativePromise(x) || hasPromiseAPI(x);
 is.generator = x => is.iterable(x) && is.function(x.next) && is.function(x.throw);
 
 // TODO: Change to use `isObjectOfType` once Node.js 6 or higher is targeted
-is.generatorFunction = x => is.function(x) && is.function(x.constructor) && x.constructor.name === 'GeneratorFunction';
+const isFunctionOfType = type => x => is.function(x) && is.function(x.constructor) && x.constructor.name === type;
+
+is.generatorFunction = isFunctionOfType('GeneratorFunction');
+is.asyncFunction = isFunctionOfType('AsyncFunction');
 
 is.regExp = isObjectOfType('RegExp');
 is.date = isObjectOfType('Date');

readme.md

@@ -84,6 +84,18 @@ Returns `true` for any object that implements its own `.next()` and `.throw()` m
 
 ##### .generatorFunction(value)
 
+##### .asyncFunction(value)
+
+Returns `true` for any `async` function that can be called with the `await` operator. 
+
+```js
+is.asyncFunction(async () => {});
+// => true
+
+is.asyncFunction(() => {});
+// => false
+```
+
 ##### .map(value)
 ##### .set(value)
 ##### .weakMap(value)

test.js

@@ -68,6 +68,10 @@ const types = new Map([
 	['generatorFunction', function * () {
 		yield 4;
 	}],
+	['asyncFunction', [
+		async function () {},
+		async () => {}
+	]],
 	['map', new Map()],
 	['set', new Set()],
 	['weakMap', new WeakMap()],
@@ -172,7 +176,7 @@ test('is.array', t => {
 });
 
 test('is.function', t => {
-	testType(t, 'function', ['generatorFunction']);
+	testType(t, 'function', ['generatorFunction', 'asyncFunction']);
 });
 
 test('is.buffer', t => {
@@ -215,6 +219,12 @@ test('is.generatorFunction', t => {
 	testType(t, 'generatorFunction', ['function']);
 });
 
+if (isNode8orHigher) {
+	test('is.asyncFunction', t => {
+		testType(t, 'asyncFunction', ['function']);
+	});
+}
+
 test('is.map', t => {
 	testType(t, 'map');
 });