From 1ecc049c78849ee5b8a9bf9f55ca221487c847d7 Mon Sep 17 00:00:00 2001
From: Glen Keane <glenkeane.94@gmail.com>
Date: Thu, 8 Oct 2026 13:31:50 +0100
Subject: [PATCH] fix: reject invalid functions and asynchronous predicates

---
 test/test-validation.js | 110 ++++++++++++++++++++++++++++++++++++++++
 when.js                 |  15 ++++++
 2 files changed, 125 insertions(+)
 create mode 100644 test/test-validation.js

diff --git a/test/test-validation.js b/test/test-validation.js
new file mode 100644
index 0000000..c8261da
--- /dev/null
+++ b/test/test-validation.js
@@ -0,0 +1,110 @@
+const test = require("node:test");
+const assert = require("node:assert/strict");
+const { setImmediate } = require("node:timers/promises");
+const when = require("../");
+
+test("rejects invalid callbacks before evaluating the condition", () => {
+  for (const value of [undefined, null, false, 1, "callback", {}]) {
+    let checks = 0;
+    assert.throws(() => when(() => checks++, value), {
+      name: "TypeError",
+      message: "code must be a function",
+    });
+    assert.equal(checks, 0);
+  }
+});
+
+test("rejects invalid conditions with a named error", () => {
+  for (const value of [undefined, null, false, 1, "condition", {}]) {
+    assert.throws(() => when(value, () => {}), {
+      name: "TypeError",
+      message: "condition must be a function",
+    });
+  }
+});
+
+test("invalid setters preserve the previous functions", (t) => {
+  let calls = 0;
+  const handle = when(
+    () => true,
+    () => calls++,
+  );
+  t.after(() => handle.clear());
+
+  assert.throws(
+    () => handle.setCondition(null),
+    /condition must be a function/,
+  );
+  assert.throws(() => handle.setCode(null), /code must be a function/);
+  handle.reset();
+
+  assert.equal(calls, 2);
+});
+
+test("rejects Promise and callable thenable results without invoking code", async () => {
+  const callable = Object.assign(() => {}, {
+    then: (resolve) => resolve(false),
+  });
+  for (const condition of [
+    async () => false,
+    async () => {
+      throw new Error("predicate rejected");
+    },
+    () => ({ then: (resolve) => resolve(false) }),
+    () => callable,
+  ]) {
+    let calls = 0;
+    assert.throws(() => when(condition, () => calls++), {
+      name: "TypeError",
+      message: "condition must return a synchronous value",
+    });
+    assert.equal(calls, 0);
+  }
+  await setImmediate();
+});
+
+test("a non-callable then property remains an ordinary truthy value", () => {
+  let calls = 0;
+  when(
+    () => ({ then: 1 }),
+    () => calls++,
+  );
+  assert.equal(calls, 1);
+});
+
+test("reset validates the replacement predicate result", (t) => {
+  const handle = when(
+    () => false,
+    () => {},
+  );
+  t.after(() => handle.clear());
+  handle.setCondition(async () => false);
+  assert.throws(
+    () => handle.reset(),
+    /condition must return a synchronous value/,
+  );
+});
+
+test("synchronous predicate and callback errors retain their identity", () => {
+  const failure = new Error("original failure");
+  assert.throws(
+    () =>
+      when(
+        () => {
+          throw failure;
+        },
+        () => {},
+      ),
+    (error) => error === failure,
+  );
+  assert.throws(
+    () =>
+      when(
+        () => true,
+        () => {
+          throw failure;
+        },
+      ),
+    (error) => error === failure,
+  );
+});
diff --git a/when.js b/when.js
index 84b612a..a54fe42 100644
--- a/when.js
+++ b/when.js
@@ -1,13 +1,20 @@
 require("setimmediate");
 
 function when(condition, code) {
+  assertFunction(condition, "condition");
+  assertFunction(code, "code");
   let immediate;
   let generation = 0;
 
   function checkCondition() {
     const currentGeneration = generation;
     const ready = condition();
+    const asynchronous = ready != null && typeof ready.then === "function";
+    if (asynchronous) Promise.resolve(ready).catch(() => {});
     if (currentGeneration !== generation) return;
+    if (asynchronous) {
+      throw new TypeError("condition must return a synchronous value");
+    }
 
     if (ready) {
       clear();
@@ -28,10 +35,12 @@ function when(condition, code) {
   }
 
   function setCondition(newCondition) {
+    assertFunction(newCondition, "condition");
     condition = newCondition;
   }
 
   function setCode(newCode) {
+    assertFunction(newCode, "code");
     code = newCode;
   }
 
@@ -39,4 +48,10 @@ function when(condition, code) {
   return { clear, reset, setCondition, setCode };
 }
 
+function assertFunction(value, name) {
+  if (typeof value !== "function") {
+    throw new TypeError(`${name} must be a function`);
+  }
+}
+
 module.exports = when;