3 files changed, 85 insertions, 0 deletions
diff --git a/js/src/builtin/Promise.cpp b/js/src/builtin/Promise.cpp
index ec7845e89d..1d068f8c64 100644
--- a/js/src/builtin/Promise.cpp
+++ b/js/src/builtin/Promise.cpp
@@ -2006,6 +2006,13 @@ CommonStaticResolveRejectImpl(JSContext* cx, HandleValue thisVal, HandleValue ar
     return promise;
 }
 
+MOZ_MUST_USE JSObject*
+js::PromiseResolve(JSContext* cx, HandleObject constructor, HandleValue value)
+{
+    RootedValue C(cx, ObjectValue(*constructor));
+    return CommonStaticResolveRejectImpl(cx, C, value, ResolveMode);
+}
+
 /**
  * ES2016, 25.4.4.4, Promise.reject.
  */
@@ -2739,6 +2746,7 @@ CreatePromisePrototype(JSContext* cx, JSProtoKey key)
 static const JSFunctionSpec promise_methods[] = {
     JS_SELF_HOSTED_FN("catch", "Promise_catch", 1, 0),
     JS_FN("then", Promise_then, 2, 0),
+    JS_SELF_HOSTED_FN("finally", "Promise_finally", 1, 0),
     JS_FS_END
 };
 
diff --git a/js/src/builtin/Promise.h b/js/src/builtin/Promise.h
index c76dc358c7..6a6453e465 100644
--- a/js/src/builtin/Promise.h
+++ b/js/src/builtin/Promise.h
@@ -128,6 +128,14 @@ OriginalPromiseThen(JSContext* cx, Handle<PromiseObject*> promise,
                     HandleValue onFulfilled, HandleValue onRejected,
                     MutableHandleObject dependent, bool createDependent);
 
+/**
+ * PromiseResolve ( C, x )
+ *
+ * The abstract operation PromiseResolve, given a constructor and a value,
+ * returns a new promise resolved with that value.
+ */
+MOZ_MUST_USE JSObject*
+PromiseResolve(JSContext* cx, HandleObject constructor, HandleValue value);
 
 MOZ_MUST_USE PromiseObject*
 CreatePromiseObjectForAsync(JSContext* cx, HandleValue generatorVal);
diff --git a/js/src/builtin/Promise.js b/js/src/builtin/Promise.js
index 704cbab0be..91a1e1f562 100644
--- a/js/src/builtin/Promise.js
+++ b/js/src/builtin/Promise.js
@@ -14,3 +14,72 @@ function Promise_catch(onRejected) {
     // Steps 1-2.
     return callContentFunction(this.then, this, undefined, onRejected);
 }
+
+// Promise.prototype.finally(onFinally)
+// See https://tc39.es/proposal-promise-finally/
+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 inferring of a function name.
+        (thenFinally) = function(value) {
+            // Steps 1-2 (implicit).
+
+            // Step 3.
+            var result = onFinally();
+
+            // 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.
+        // The parentheses prevent the inferring of a function name.
+        (catchFinally) = function(reason) {
+            // Steps 1-2 (implicit).
+
+            // Step 3.
+            var result = onFinally();
+
+            // 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);
+}