3 files changed, 66 insertions, 0 deletions
diff --git a/js/libdeno.ts b/js/libdeno.ts
index 97269c36c..842b0c8ad 100644
--- a/js/libdeno.ts
+++ b/js/libdeno.ts
@@ -3,6 +3,12 @@ import { globalEval } from "./global_eval";
 
 // The libdeno functions are moved so that users can't access them.
 type MessageCallback = (msg: Uint8Array) => void;
+export type PromiseRejectEvent =
+  | "RejectWithNoHandler"
+  | "HandlerAddedAfterReject"
+  | "ResolveAfterResolved"
+  | "RejectAfterResolved";
+
 interface Libdeno {
   recv(cb: MessageCallback): void;
 
@@ -20,6 +26,17 @@ interface Libdeno {
     ) => void
   ) => void;
 
+  setPromiseRejectHandler: (
+    handler: (
+      error: Error | string,
+      event: PromiseRejectEvent,
+      /* tslint:disable-next-line:no-any */
+      promise: Promise<any>
+    ) => void
+  ) => void;
+
+  setPromiseErrorExaminer: (handler: () => boolean) => void;
+
   mainSource: string;
   mainSourceMap: RawSourceMap;
 }
diff --git a/js/main.ts b/js/main.ts
index 24418b53e..0d33cf063 100644
--- a/js/main.ts
+++ b/js/main.ts
@@ -7,6 +7,7 @@ import { DenoCompiler } from "./compiler";
 import { libdeno } from "./libdeno";
 import { args } from "./deno";
 import { sendSync, handleAsyncMsgFromRust } from "./dispatch";
+import { promiseErrorExaminer, promiseRejectHandler } from "./promise_util";
 
 function sendStart(): msg.StartRes {
   const builder = new flatbuffers.Builder();
@@ -39,6 +40,8 @@ function onGlobalError(
 export default function denoMain() {
   libdeno.recv(handleAsyncMsgFromRust);
   libdeno.setGlobalErrorHandler(onGlobalError);
+  libdeno.setPromiseRejectHandler(promiseRejectHandler);
+  libdeno.setPromiseErrorExaminer(promiseErrorExaminer);
   const compiler = DenoCompiler.instance();
 
   // First we send an empty "Start" message to let the privileged side know we
diff --git a/js/promise_util.ts b/js/promise_util.ts
new file mode 100644
index 000000000..a550bc388
--- /dev/null
+++ b/js/promise_util.ts
@@ -0,0 +1,46 @@
+import { PromiseRejectEvent } from "./libdeno";
+
+/* tslint:disable-next-line:no-any */
+const rejectMap = new Map<Promise<any>, string>();
+// For uncaught promise rejection errors
+
+/* tslint:disable-next-line:no-any */
+const otherErrorMap = new Map<Promise<any>, string>();
+// For reject after resolve / resolve after resolve errors
+
+export function promiseRejectHandler(
+  error: Error | string,
+  event: PromiseRejectEvent,
+  /* tslint:disable-next-line:no-any */
+  promise: Promise<any>
+) {
+  switch (event) {
+    case "RejectWithNoHandler":
+      rejectMap.set(promise, (error as Error).stack || "RejectWithNoHandler");
+      break;
+    case "HandlerAddedAfterReject":
+      rejectMap.delete(promise);
+      break;
+    default:
+      // error is string here
+      otherErrorMap.set(promise, `Promise warning: ${error as string}`);
+  }
+}
+
+// Return true when continue, false to die on uncaught promise reject
+export function promiseErrorExaminer(): boolean {
+  if (otherErrorMap.size > 0) {
+    for (const msg of otherErrorMap.values()) {
+      console.log(msg);
+    }
+    otherErrorMap.clear();
+  }
+  if (rejectMap.size > 0) {
+    for (const msg of rejectMap.values()) {
+      console.log(msg);
+    }
+    rejectMap.clear();
+    return false;
+  }
+  return true;
+}