Explain how the `process.nextTick()` function in Node.js affects the order of execution within the event loop, and in what scenarios is it critical?

`process.nextTick()` allows you to schedule a callback function to be executed after the current operation completes but *beforethe event loop continues to its next iteration. This means the callback is executed before any I/O events, timers, or other callbacks in the task queue. It essentially defers the execution of a function until the next tick of the event loop. When `process.nextTick(callback)` is called, the `callback` is added to a special queue called the 'next tick queue'. After the current operation on the call stack finishes executing, the event loop will check the next tick queue. If there are any callbacks in this queue, they will be executed *beforethe event loop moves on to process any other tasks in the regular task queue (like I/O callbacks from file reads or network requests). This is different from `setTimeout(callback, 0)`, which adds the callback to the *regular task queueand will be executed in a later iteration of the event loop. Scenarios where `process.nextTick()` is critical include: 1. Deferring the execution of a callback to ensure it runs *afterthe current call stack has unwound, preventing stack overflows. 2. Ensuring that a callback is executed *beforeany I/O events are processed, providing a way to prioritize certain operations. 3. Adhering to API design contracts where a callback is expected to be executed asynchronously, even if the operation could be completed synchronously. For example, if a function sometimes performs a synchronous operation and sometimes an asynchronous operation, using `process.nextTick()` ensures that the callback is *alwaysinvoked asynchronously, maintaining consistent behavior. However, it's crucial to use `process.nextTick()` judiciously, as repeatedly adding callbacks to the next tick queue can starve the event loop and prevent it from processing I/O events, leading to performance issues. It should be used for immediate deferral of execution within the same event loop iteration, not as a general-purpose scheduling mechanism.