perf: don't run microtask checkpoint if macrotask callback did no work (#19492)

Most of the time there's no firing timers, nor pending promise
rejections, so it's wasteful to run microtask checkpoint 
additionally twice on each tick of the event loop.

Closes https://github.com/denoland/deno/issues/18871
Ref https://github.com/denoland/deno/issues/19451

1 files changed, 3 insertions, 1 deletions

diff --git a/ext/web/02_timers.js b/ext/web/02_timers.js
index 19ebfaa0e..27e2e953d 100644
--- a/ext/web/02_timers.js
+++ b/ext/web/02_timers.js
@@ -54,8 +54,10 @@ const timerTasks = [];
 let timerNestingLevel = 0;
 
 function handleTimerMacrotask() {
+  // We have no work to do, tell the runtime that we don't
+  // need to perform microtask checkpoint.
   if (timerTasks.length === 0) {
-    return true;
+    return undefined;
   }
 
   const task = ArrayPrototypeShift(timerTasks);