From f83d5fcfd994f8efb48846e4911ded8f71388af3 Mon Sep 17 00:00:00 2001
From: acandoo <117209328+acandoo@users.noreply.github.com>
Date: Fri, 27 Feb 2026 19:28:28 +0000
Subject: [PATCH 1/2] Handle unhandled Promise rejection in JavaScript

---
 src/gleeunit_ffi.mjs | 39 +++++++++++++++++++++++++++++++++++++--
 1 file changed, 37 insertions(+), 2 deletions(-)

diff --git a/src/gleeunit_ffi.mjs b/src/gleeunit_ffi.mjs
index 7bdc071..0150627 100644
--- a/src/gleeunit_ffi.mjs
+++ b/src/gleeunit_ffi.mjs
@@ -40,6 +40,21 @@ export async function main() {
   let packageName = await readRootPackageName();
   let dist = `../${packageName}/`;
 
+  // Since we want to be able to report unhandled Promise rejections as test failures,
+  // we need to keep track of them as they happen. This is because they can occur at any
+  // time during the test run, and we want to be able to report them at the end of the test run,
+  // after all tests have been executed.
+  const rejections = [];
+
+  // Node's approach to unhandled rejections is different
+  // from the Web/Deno API, in that attaching an event listener
+  // through `globalThis.addEventListener` or `globalThis.onunhandledrejection`
+  // still causes the process to prematurely exit. Instead, we have to use `process.on`,
+  // as it will allow us to manually handle the exiting behavior.
+  globalThis.process?.on("unhandledRejection", (reason) => {
+    rejections.push(reason);
+  });
+
   for await (let path of await gleamFiles("test")) {
     let js_path = path.slice("test/".length).replace(".gleam", ".mjs");
     let module = await import(join_path(dist, js_path));
@@ -55,8 +70,28 @@ export async function main() {
     }
   }
 
-  const status = reporting.finished(state);
-  exit(status);
+  const preExit = () => {
+    // While a likely module/function can be inferred from what test is
+    // currently running, unhandled Promise rejections outlast the lifetime
+    // of ES module dynamic import or function calls, which means that they
+    // can reject on any module/function running.
+    //
+    // We also put this logic at the end of the test runner so that we can run all tests.
+    for (const entry of rejections) {
+      state = reporting.test_failed(state, "Unknown module", "Unknown function", entry);
+    }
+
+    const status = reporting.finished(state);
+    exit(status);
+  };
+
+  // If we're able to subscribe to beforeExit, then we can report unhandled rejections that
+  // occur after the tests are finished.
+  if (globalThis.process?.on) {
+    process.on("beforeExit", preExit);
+  } else {
+    preExit();
+  }
 }
 
 export function crash(message) {

From 3a0b018227428a5da50e1a2b3d6777830c29a704 Mon Sep 17 00:00:00 2001
From: acandoo <117209328+acandoo@users.noreply.github.com>
Date: Sat, 14 Mar 2026 04:51:55 +0000
Subject: [PATCH 2/2] Add test for unhandled Promise rejection

---
 gleam.toml                   |  2 ++
 manifest.toml                |  4 +++
 test/gleam_panics_test.gleam | 64 ++++++++++++++++++++++++++++++++++++
 test/gleeunit_test_ffi.mjs   | 19 +++++++++++
 4 files changed, 89 insertions(+)

diff --git a/gleam.toml b/gleam.toml
index c7df0c5..9296023 100644
--- a/gleam.toml
+++ b/gleam.toml
@@ -14,3 +14,5 @@ gleam_stdlib = ">= 0.60.0 and < 1.0.0"
 
 [dev-dependencies]
 testhelper = { "path" = "./testhelper" }
+shellout = ">= 1.8.0 and < 2.0.0"
+envoy = ">= 1.1.0 and < 2.0.0"
diff --git a/manifest.toml b/manifest.toml
index f73875e..65c0d15 100644
--- a/manifest.toml
+++ b/manifest.toml
@@ -2,10 +2,14 @@
 # You typically do not need to edit this file
 
 packages = [
+  { name = "envoy", version = "1.1.0", build_tools = ["gleam"], requirements = ["gleam_stdlib"], otp_app = "envoy", source = "hex", outer_checksum = "850DA9D29D2E5987735872A2B5C81035146D7FE19EFC486129E44440D03FD832" },
   { name = "gleam_stdlib", version = "0.60.0", build_tools = ["gleam"], requirements = [], otp_app = "gleam_stdlib", source = "hex", outer_checksum = "621D600BB134BC239CB2537630899817B1A42E60A1D46C5E9F3FAE39F88C800B" },
+  { name = "shellout", version = "1.8.0", build_tools = ["gleam"], requirements = ["gleam_stdlib"], otp_app = "shellout", source = "hex", outer_checksum = "C416356D45151F298108C9DB9CD1EDE0313F620B5EDBB5766CD7237659D87841" },
   { name = "testhelper", version = "1.0.0", build_tools = ["gleam"], requirements = ["gleam_stdlib"], source = "local", path = "testhelper" },
 ]
 
 [requirements]
+envoy = { version = ">= 1.1.0 and < 2.0.0" }
 gleam_stdlib = { version = ">= 0.60.0 and < 1.0.0" }
+shellout = { version = ">= 1.8.0 and < 2.0.0" }
 testhelper = { path = "./testhelper" }
diff --git a/test/gleam_panics_test.gleam b/test/gleam_panics_test.gleam
index 3f5d54d..18a879b 100644
--- a/test/gleam_panics_test.gleam
+++ b/test/gleam_panics_test.gleam
@@ -1,9 +1,11 @@
+import envoy
 import gleam/dynamic
 import gleam/function
 import gleeunit/internal/gleam_panic.{
   Assert, BinaryOperator, Expression, FunctionCall, LetAssert, Literal,
   OtherExpression, Panic, Todo, Unevaluated,
 }
+import shellout
 import testhelper
 
 @external(erlang, "gleeunit_test_ffi", "rescue")
@@ -206,3 +208,65 @@ pub fn assert_binary_operator_test() {
   assert right.end == end
   assert right.kind == Unevaluated
 }
+
+pub fn javascript_unhandled_promise_test() {
+  case envoy.get("GLEEUNIT_RUN_UNHANDLED_PROMISE_TEST") {
+    Error(_) -> {
+      // Spawn gleam test
+      let assert Error(#(code, _)) =
+        shellout.command(
+          run: "gleam",
+          with: ["test", "-t", "javascript"],
+          in: ".",
+          opt: [
+            shellout.SetEnvironment([
+              #("GLEEUNIT_RUN_UNHANDLED_PROMISE_TEST", "1"),
+            ]),
+          ],
+        )
+        as "immediate rejected promise: expected error due to unhandled promise rejection"
+
+      assert code == 1
+        as "immediate rejected promise: expected exit code 1 due to unhandled promise rejection"
+
+      // Spawn gleam test
+      let assert Error(#(code, _)) =
+        shellout.command(
+          run: "gleam",
+          with: ["test", "-t", "javascript"],
+          in: ".",
+          opt: [
+            shellout.SetEnvironment([
+              #("GLEEUNIT_RUN_UNHANDLED_PROMISE_TEST", "2"),
+            ]),
+          ],
+        )
+        as "delayed rejected promise: expected error due to unhandled promise rejection"
+
+      assert code == 1
+        as "delayed rejected promise: expected exit code 1 due to unhandled promise rejection"
+    }
+    Ok("1") -> {
+      echo "inside spawned gleam test, testing immediate reject"
+      javascript_test_promise()
+    }
+    Ok(_) -> {
+      echo "inside spawned gleam test, testing delayed reject"
+      javascript_test_delayed_promise()
+    }
+  }
+}
+
+@external(javascript, "./gleeunit_test_ffi.mjs", "promise_fail_test")
+fn javascript_test_promise() -> Nil {
+  // Not relevant to other targets
+  echo "This is not relevant to other targets!"
+  Nil
+}
+
+@external(javascript, "./gleeunit_test_ffi.mjs", "delayed_promise_fail_test")
+fn javascript_test_delayed_promise() -> Nil {
+  // Not relevant to other targets
+  echo "This is not relevant to other targets!"
+  Nil
+}
diff --git a/test/gleeunit_test_ffi.mjs b/test/gleeunit_test_ffi.mjs
index 783209d..24e1875 100644
--- a/test/gleeunit_test_ffi.mjs
+++ b/test/gleeunit_test_ffi.mjs
@@ -1,5 +1,10 @@
 import { Ok, Error } from "./gleam.mjs";
 
+// The maximum amount of time running all tests is expected to take.
+// This is used to set the timeout for the delayed promise test, to ensure
+// it runs after all other tests have had a chance to execute.
+const MAX_TEST_TIME = 4000;
+
 export function rescue(f) {
   try {
     return new Ok(f());
@@ -7,3 +12,17 @@ export function rescue(f) {
     return new Error(e);
   }
 }
+
+export function promise_fail_test() {
+  new Promise(() => {
+    throw new Error("Promise panicked");
+  });
+}
+
+export function delayed_promise_fail_test() {
+  new Promise((_, reject) => {
+    setTimeout(() => {
+      reject(new Error("Promise panicked"));
+    }, MAX_TEST_TIME);
+  });
+}