1 files changed, 78 insertions, 0 deletions
diff --git a/js/src/builtin/Promise.js b/js/src/builtin/Promise.js
new file mode 100644
index 0000000000..202ae33ad8
--- /dev/null
+++ b/js/src/builtin/Promise.js
@@ -0,0 +1,78 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Promise.prototype.finally proposal, stage 3.
+// Promise.prototype.finally ( onFinally )
+function Promise_finally(onFinally) {
+  // Step 1.
+  var promise = this;
+
+  // Step 2.
+  if (!IsObject(promise)) {
+    ThrowTypeError(JSMSG_INCOMPATIBLE_PROTO, "Promise", "finally", "value");
+  }
+
+  // Step 3.
+  var C = SpeciesConstructor(promise, GetBuiltinConstructor("Promise"));
+
+  // Step 4.
+  assert(IsConstructor(C), "SpeciesConstructor returns a constructor function");
+
+  // Steps 5-6.
+  var thenFinally, catchFinally;
+  if (!IsCallable(onFinally)) {
+    thenFinally = onFinally;
+    catchFinally = onFinally;
+  } else {
+    // ThenFinally Function.
+    // The parentheses prevent the infering of a function name.
+    // eslint-disable-next-line prettier/prettier
+    (thenFinally) = function(value) {
+      // Steps 1-2 (implicit).
+
+      // Step 3.
+      var result = callContentFunction(onFinally, undefined);
+
+      // Steps 4-5 (implicit).
+
+      // Step 6.
+      var promise = PromiseResolve(C, result);
+
+      // Step 7.
+      // FIXME: spec issue - "be equivalent to a function that" is not a defined spec term.
+      // https://github.com/tc39/ecma262/issues/933
+
+      // Step 8.
+      return callContentFunction(promise.then, promise, function() {
+        return value;
+      });
+    };
+
+    // CatchFinally Function.
+    // eslint-disable-next-line prettier/prettier
+    (catchFinally) = function(reason) {
+      // Steps 1-2 (implicit).
+
+      // Step 3.
+      var result = callContentFunction(onFinally, undefined);
+
+      // Steps 4-5 (implicit).
+
+      // Step 6.
+      var promise = PromiseResolve(C, result);
+
+      // Step 7.
+      // FIXME: spec issue - "be equivalent to a function that" is not a defined spec term.
+      // https://github.com/tc39/ecma262/issues/933
+
+      // Step 8.
+      return callContentFunction(promise.then, promise, function() {
+        throw reason;
+      });
+    };
+  }
+
+  // Step 7.
+  return callContentFunction(promise.then, promise, thenFinally, catchFinally);
+}