If we execute setTimeout with a delay of 300 milliseconds, after how much time will the callback actually execute?

setTimeout with a delay of 300 milliseconds guarantees that the callback will be called no earlier than 300 ms, but the actual execution time may be longer due to the characteristics of the JavaScript engine and event loop.

JavaScript is single-threaded, and if at the moment the timer triggers the main thread is busy executing other code, the callback will be queued and executed immediately after the thread is free. Thus, the delay can be more than 300 ms, but not less.

Example:

setTimeout(() => {
  console.log('Executed after at least 300 ms');
}, 300);

If the main thread is busy with heavy computations at that time, the callback will execute later.