The answer
No. The callback never runs before the delay. It may run later (if the stack is busy, microtasks are queued, or the tab is in the background), but never earlier.
setTimeout Interview Questions: Trust Issues in JavaScript
Can a setTimeout callback run before the specified delay?
More FAQs in setTimeout Interview Questions: Trust Issues in JavaScript
No. It guarantees a minimum of 1000ms. The actual delay depends on the call stack, microtask queue, background tab throttling, and nested timer clamping. The callback runs at least after 1000ms, but often later.
4ms after 5 nested levels. The HTML spec specifies this to prevent abuse. The first 5 levels can use 0ms, but after that, the delay is clamped to 4ms.
The delay is treated as 0. The callback goes to the macrotask queue and runs after the call stack is empty and all microtasks are done.
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