diff --git a/crates/futures/src/atomics.rs b/crates/futures/src/atomics.rs
index 61481707..5a5e1dd4 100644
--- a/crates/futures/src/atomics.rs
+++ b/crates/futures/src/atomics.rs
@@ -1,5 +1,6 @@
 use std::cell::{Cell, RefCell};
 use std::fmt;
+use std::rc::Rc;
 use std::sync::atomic::{AtomicBool, AtomicI32, Ordering};
 use std::sync::Arc;
 
@@ -19,9 +20,7 @@ use wasm_bindgen::prelude::*;
 ///
 /// Currently this type is constructed with `JsFuture::from`.
 pub struct JsFuture {
-    resolved: oneshot::Receiver<JsValue>,
-    rejected: oneshot::Receiver<JsValue>,
-    callbacks: Option<(Closure<dyn FnMut(JsValue)>, Closure<dyn FnMut(JsValue)>)>,
+    rx: oneshot::Receiver<Result<JsValue, JsValue>>,
 }
 
 impl fmt::Debug for JsFuture {
@@ -33,28 +32,49 @@ impl fmt::Debug for JsFuture {
 impl From<Promise> for JsFuture {
     fn from(js: Promise) -> JsFuture {
         // Use the `then` method to schedule two callbacks, one for the
-        // resolved value and one for the rejected value. These two callbacks
-        // will be connected to oneshot channels which feed back into our
-        // future.
+        // resolved value and one for the rejected value. We're currently
+        // assuming that JS engines will unconditionally invoke precisely one of
+        // these callbacks, no matter what.
         //
-        // This may not be the speediest option today but it should work!
-        let (tx1, rx1) = oneshot::channel();
-        let (tx2, rx2) = oneshot::channel();
-        let mut tx1 = Some(tx1);
-        let resolve = Closure::wrap(Box::new(move |val| {
-            drop(tx1.take().unwrap().send(val));
-        }) as Box<dyn FnMut(_)>);
-        let mut tx2 = Some(tx2);
-        let reject = Closure::wrap(Box::new(move |val| {
-            drop(tx2.take().unwrap().send(val));
-        }) as Box<dyn FnMut(_)>);
+        // Ideally we'd have a way to cancel the callbacks getting invoked and
+        // free up state ourselves when this `JsFuture` is dropped. We don't
+        // have that, though, and one of the callbacks is likely always going to
+        // be invoked.
+        //
+        // As a result we need to make sure that no matter when the callbacks
+        // are invoked they are valid to be called at any time, which means they
+        // have to be self-contained. Through the `Closure::once` and some
+        // `Rc`-trickery we can arrange for both instances of `Closure`, and the
+        // `Rc`, to all be destroyed once the first one is called.
+        let (tx, rx) = oneshot::channel();
+        let state = Rc::new(RefCell::new(None));
+        let state2 = state.clone();
+        let resolve = Closure::once(move |val| finish(&state2, Ok(val)));
+        let state2 = state.clone();
+        let reject = Closure::once(move |val| finish(&state2, Err(val)));
 
         js.then2(&resolve, &reject);
+        *state.borrow_mut() = Some((tx, resolve, reject));
 
-        JsFuture {
-            resolved: rx1,
-            rejected: rx2,
-            callbacks: Some((resolve, reject)),
+        return JsFuture { rx };
+
+        fn finish(
+            state: &RefCell<
+                Option<(
+                    oneshot::Sender<Result<JsValue, JsValue>>,
+                    Closure<dyn FnMut(JsValue)>,
+                    Closure<dyn FnMut(JsValue)>,
+                )>,
+            >,
+            val: Result<JsValue, JsValue>,
+        ) {
+            match state.borrow_mut().take() {
+                // We don't have any guarantee that anyone's still listening at this
+                // point (the Rust `JsFuture` could have been dropped) so simply
+                // ignore any errors here.
+                Some((tx, _, _)) => drop(tx.send(val)),
+                None => wasm_bindgen::throw_str("cannot finish twice"),
+            }
         }
     }
 }
@@ -64,19 +84,11 @@ impl Future for JsFuture {
     type Error = JsValue;
 
     fn poll(&mut self) -> Poll<JsValue, JsValue> {
-        // Test if either our resolved or rejected side is finished yet. Note
-        // that they will return errors if they're disconnected which can't
-        // happen until we drop the `callbacks` field, which doesn't happen
-        // till we're done, so we dont need to handle that.
-        if let Ok(Async::Ready(val)) = self.resolved.poll() {
-            drop(self.callbacks.take());
-            return Ok(val.into());
+        match self.rx.poll() {
+            Ok(Async::Ready(val)) => val.map(Async::Ready),
+            Ok(Async::NotReady) => Ok(Async::NotReady),
+            Err(_) => wasm_bindgen::throw_str("cannot cancel"),
         }
-        if let Ok(Async::Ready(val)) = self.rejected.poll() {
-            drop(self.callbacks.take());
-            return Err(val);
-        }
-        Ok(Async::NotReady)
     }
 }
 
@@ -101,8 +113,8 @@ impl Future for JsFuture {
 /// resolve**. Instead it will be a leaked promise. This is an unfortunate
 /// limitation of wasm currently that's hoped to be fixed one day!
 pub fn future_to_promise<F>(future: F) -> Promise
-where
-    F: Future<Item = JsValue, Error = JsValue> + 'static,
+    where
+        F: Future<Item = JsValue, Error = JsValue> + 'static,
 {
     _future_to_promise(Box::new(future))
 }
@@ -283,9 +295,7 @@ fn _future_to_promise(future: Box<dyn Future<Item = JsValue, Error = JsValue>>)
                         break;
                     }
 
-                    State::Waiting(_) => {
-                        panic!("shouldn't see waiting state!")
-                    }
+                    State::Waiting(_) => panic!("shouldn't see waiting state!"),
                 }
 
                 let (val, f) = match me.spawn.borrow_mut().poll_future_notify(&me.waker, 0) {
@@ -315,8 +325,8 @@ fn _future_to_promise(future: Box<dyn Future<Item = JsValue, Error = JsValue>>)
 ///
 /// This function has the same panic behavior as `future_to_promise`.
 pub fn spawn_local<F>(future: F)
-where
-    F: Future<Item = (), Error = ()> + 'static,
+    where
+        F: Future<Item = (), Error = ()> + 'static,
 {
     future_to_promise(
         future