From ba4a33847e8a9401b7e6ebb4f850418c096474d1 Mon Sep 17 00:00:00 2001
From: James Garbutt <43081j@users.noreply.github.com>
Date: Thu, 17 Sep 2026 13:44:42 +0100
Subject: [PATCH] fix(prefer-timer-args): check UpdateExpressions &
 TemplateLiterals

An `UpdateExpression` is a mutation so we can't safely inline it. For
example:

```ts
setTimeout(() => fn(i++), 10);
// not the same as
setTimeout(fn, 10, i++);
```

A `TemplateLiteral` may contain expressions which mutate, so we also now
check those:

```ts
setTimeout(() => `foo ${fn()} bar`, 10);
```
---
 src/rules/prefer-timer-args.test.ts | 10 ++++++++++
 src/rules/prefer-timer-args.ts      |  8 ++++++--
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/src/rules/prefer-timer-args.test.ts b/src/rules/prefer-timer-args.test.ts
index dc5b8f1..632d5e9 100644
--- a/src/rules/prefer-timer-args.test.ts
+++ b/src/rules/prefer-timer-args.test.ts
@@ -51,6 +51,16 @@ ruleTester.run('prefer-timer-args', preferTimerArgs, {
     'setTimeout(() => fn({key: getData()}), 100)',
     'setTimeout(fn.bind(null, getData()), 100)',
 
+    // Unsafe transformations - arguments mutate state
+    'setTimeout(() => fn(i++), 100)',
+    'setTimeout(() => fn(--i), 100)',
+    'setTimeout(() => fn(delete obj.key), 100)',
+    'setTimeout(fn.bind(null, i++), 100)',
+
+    // Unsafe transformations - template literals containing calls
+    'setTimeout(() => fn(`${getData()}`), 100)',
+    'setTimeout(() => fn(`${i++}`), 100)',
+
     // window.setTimeout examples
     'window.setTimeout(doSomething, 100)',
     'window.setTimeout(() => { doSomething(a); }, 100)',
diff --git a/src/rules/prefer-timer-args.ts b/src/rules/prefer-timer-args.ts
index ffed63b..bdd0d0a 100644
--- a/src/rules/prefer-timer-args.ts
+++ b/src/rules/prefer-timer-args.ts
@@ -37,9 +37,11 @@ function isSafeArgument(arg: Expression | SpreadElement): boolean {
   switch (arg.type) {
     case 'Identifier':
     case 'Literal':
-    case 'TemplateLiteral':
       return true;
 
+    case 'TemplateLiteral':
+      return arg.expressions.every(isSafeArgument);
+
     case 'MemberExpression':
       if (
         arg.object.type === 'Super' ||
@@ -76,7 +78,9 @@ function isSafeArgument(arg: Expression | SpreadElement): boolean {
       });
 
     case 'UnaryExpression':
-    case 'UpdateExpression':
+      if (arg.operator === 'delete') {
+        return false;
+      }
       return isSafeArgument(arg.argument);
 
     case 'BinaryExpression':