Add Promise finally method to polyfill

promise/promise.js

@@ -61,6 +61,20 @@ PromisePolyfill.prototype.then = function(onFulfilled, onRejection) {
 PromisePolyfill.prototype.catch = function(onRejection) {
 	return this.then(null, onRejection)
 }
+PromisePolyfill.prototype.finally = function(callback) {
+	return this.then(
+		function(value) {
+			return PromisePolyfill.resolve(callback()).then(function() {
+				return value
+			})
+		},
+		function(reason) {
+			return PromisePolyfill.resolve(callback()).then(function() {
+				return PromisePolyfill.reject(reason);
+			})
+		}
+	)
+}
 PromisePolyfill.resolve = function(value) {
 	if (value instanceof PromisePolyfill) return value
 	return new PromisePolyfill(function(resolve) {resolve(value)})
@@ -96,10 +110,18 @@ PromisePolyfill.race = function(list) {
 }
 
 if (typeof window !== "undefined") {
-	if (typeof window.Promise === "undefined") window.Promise = PromisePolyfill
+	if (typeof window.Promise === "undefined") {
+		window.Promise = PromisePolyfill
+	} else if (!window.Promise.prototype.finally) {
+		window.Promise.prototype.finally = PromisePolyfill.prototype.finally
+	}
 	module.exports = window.Promise
 } else if (typeof global !== "undefined") {
-	if (typeof global.Promise === "undefined") global.Promise = PromisePolyfill
+	if (typeof global.Promise === "undefined") {
+		global.Promise = PromisePolyfill
+	} else if (!global.Promise.prototype.finally) {
+		global.Promise.prototype.finally = PromisePolyfill.prototype.finally
+	}
 	module.exports = global.Promise
 } else {
 	module.exports = PromisePolyfill

promise/tests/test-promise.js

@@ -15,6 +15,7 @@ o.spec("promise", function() {
 		o("constructor has correct methods", function() {
 			o(typeof Promise.prototype.then).equals("function")
 			o(typeof Promise.prototype.catch).equals("function")
+			o(typeof Promise.prototype.finally).equals("function")
 			o(typeof Promise.resolve).equals("function")
 			o(typeof Promise.reject).equals("function")
 			o(typeof Promise.race).equals("function")
@@ -53,6 +54,24 @@ o.spec("promise", function() {
 				o(value).equals(1)
 			}).then(done)
 		})
+		o("finally after then returns promise", function(done) {
+			var promise = Promise.resolve(1)
+
+			promise.then(function(value) {
+				o(value).equals(1)
+			}).finally(function(value) {
+				o(value).equals(undefined)
+			}).then(done)
+		})
+		o("finally after catch returns promise", function(done) {
+			var promise = Promise.reject(1)
+
+			promise.catch(function(value) {
+				o(value).equals(1)
+			}).finally(function(value) {
+				o(value).equals(undefined)
+			}).then(done)
+		})
 	})
 	o.spec("resolve", function() {
 		o("resolves once", function(done) {