What is JavaScript's event loop?

The pump scheduling tasks and callbacks after sync finishes. No interrupts.

Why doesn't setTimeout(0) run immediately?

It's a macrotask. Waits for current task (your loop) to empty the stack.

Nope. Syntactic nicety over Promises. Still single-task at a time.

Paste that billion-loop into your console. Fire off a setTimeout(..., 0). Watch it starve. JavaScript's runtime isn't what you think.

theAIcatchup Apr 10, 2026 3 min read

JS tasks run synchronously to completion—no interruptions from async callbacks. 𝕏
Distinguish engine (call stack) from runtime (scheduling tasks). 𝕏
Fix mental model: cooperative tasks, not pre-emptive threading. 𝕏

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#async javascript #javascript-event-loop #js runtime #single-threaded js

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Originally reported by dev.to