Mastering Event Loop: Tricky Node Interview Questions

Node.js developers preparing for interviews often face tricky execution-order questions that revolve around the behavior of the event loop. Concepts like promises, setTimeout, and setImmediate can confuse even experienced developers because of how they interact within the Node.js environment.

In this article, we’ll break down a couple of tricky event-loop related questions, explain their execution flow, and help you understand the nuances of the event loop. This knowledge is essential for performing well in interviews where understanding asynchronous behavior is crucial.

Understanding the Node.js Event Loop

Before diving into the examples, let’s review the basics of the Node.js event loop. Node.js processes events in phases:

• Synchronous Code Execution: All synchronous code is executed first here, blocking the event loop until it's complete.
• Microtask Queue (Promises & Process.nextTick): After synchronous code, Node.js checks the microtask queue. This queue includes callbacks from resolved/rejected promises and process.nextTick. Microtasks have higher priority and always run before moving to the next event loop phase.
• Timers Phase (setTimeout & setInterval): If a timer has expired, its callback is executed.
• I/O Callbacks Phase: Handles I/O operations like reading from the network or filesystem.
• Check Phase (setImmediate): Executes callbacks registered by setImmediate.
• Close Callbacks Phase: Executes close events like closing a server.

Let’s look at some questions that are commonly asked in interviews to understand how these mechanisms interact.

Question 1: Tricky Promise, setTimeout, and setImmediate Execution

console.log('Before everything');

const promise = new Promise((resolve) => {
  console.log('Promise started');
  setImmediate(() => {
    console.log('Inside setImmediate from promise');
  });
  resolve('Promise resolved');
});

promise.then((result) => {
  console.log('Promise result:', result);
  setImmediate(() => {
    console.log('Inside setImmediate from promise then');
  });
});

setTimeout(() => {
  console.log('Inside setTimeout');
}, 0);

console.log('After everything');        

Expected Output:

Before everything
Promise started
After everything
Promise result: Promise resolved
Inside setImmediate from promise
Inside setImmediate from promise then
Inside setTimeout        

Explanation:

1. Synchronous execution: The script starts with console.log('Before everything') and logs Before everything. Then the Promise is created and console.log('Promise started') is called.
2. Promise resolution: The promise is resolved (resolve('Promise resolved')), but its .then() callback goes into the microtask queue, which will run later.
3. setImmediate vs setTimeout: Both setImmediate and setTimeout are scheduled, but setImmediate runs in the check phase, while setTimeout runs in the timers phase, which happens after the check phase.
4. Microtask execution: Once the synchronous code completes (console.log('After everything')), Node.js processes the microtask queue, where the promise’s .then() handler is executed. This prints Promise result: Promise resolved.
5. Check Phase (setImmediate): Next, setImmediate callbacks are executed (Inside setImmediate from promise, Inside setImmediate from promise then).
6. Timers Phase (setTimeout): Finally, the setTimeout callback is executed (Inside setTimeout).

This sequence highlights how promises and event loop phases like timers and immediate callbacks interleave with one another.

Question 2: setImmediate vs setTimeout Inside I/O Callbacks

const fs = require('fs');

fs.readFile(__filename, () => {
  console.log('File read completed');

  setTimeout(() => {
    console.log('Inside setTimeout');
  }, 0);

  setImmediate(() => {
    console.log('Inside setImmediate');
  });
});

console.log('End of script');        

NOTE:

A notable exception to this question is, when both setTimeout and setImmediate are used within an I/O callback function, such as fs.readFile. In this scenario, the setImmediate callback is executed first, as it is scheduled to run after I/O callbacks but before timer callbacks in the event loop's phases.

This behavior can be counterintuitive, as in most cases, setTimeout would execute first when called outside of an I/O operation.

Expected Output:

End of script
File read completed
Inside setImmediate
Inside setTimeout        

Explanation:

1. Synchronous execution: First, console.log('End of script') is executed, printing End of script.
2. I/O Callback Phase: After the file is read, the callback provided to fs.readFile is executed. console.log('File read completed') is printed.
3. Check Phase (setImmediate): Inside the I/O callback, both setTimeout and setImmediate are called. However, setImmediate callbacks are always executed in the check phase, which happens right after I/O callbacks and before the timers phase. Hence Inside setImmediate is printed.
4. Timers Phase (setTimeout): Finally, the setTimeout callback is executed in the timers phase, printing Inside setTimeout.

Question 3: Promise Inside fs.readFile

const fs = require('fs');

fs.readFile(__filename, () => {
  console.log('File read completed');

  setTimeout(() => {
    console.log('Inside setTimeout');
  }, 0);

  setImmediate(() => {
    console.log('Inside setImmediate');
  });

  Promise.resolve().then(() => {
    console.log('Inside Promise');
  });
});

console.log('End of script');        

Expected Output:

End of script
File read completed
Inside Promise
Inside setImmediate
Inside setTimeout        

Explanation:

1. Synchronous execution: The script starts by executing console.log('End of script'), which prints End of script.
2. I/O Callback Phase: Once the file is read, the callback of fs.readFile is executed (File read completed).
3. Microtask Queue (Promise): After the I/O operation, the promise's .then() callback gets added to the microtask queue. Since microtasks always have a higher priority than setImmediate or setTimeout, the promise resolves immediately after the I/O callback finishes, and Inside Promise is printed.
4. Check Phase (setImmediate): After microtasks are handled, the check phase begins, where setImmediate callbacks are processed, so Inside setImmediate is printed.
5. Timers Phase (setTimeout): Finally, the timers phase runs, executing the setTimeout callback, which prints Inside setTimeout.

The reason setImmediate is executed before setTimeout here is the same, as we discussed in Question 2.

Key Takeaways for Interviews:

To ace questions related to the JavaScript event loop, especially in Node.js, here are a few important points to remember:

1. Synchronous code is always executed first. Before the event loop starts processing any tasks or callbacks, all synchronous code is executed from top to bottom.
2. Microtasks (Promises) run before macrotasks (like setTimeout and setImmediate). Promises are processed as soon as the current execution stack is cleared, which means they have priority over any other callback scheduled via setTimeout or setImmediate.
3. After I/O operations (like fs.readFile()), the check phase (where setImmediate callbacks are executed) runs before the timers phase (where setTimeout is processed), meaning setImmediate will generally be executed before setTimeout when they’re in the same I/O callback (as discussed in Question 2).
4. setTimeout and setImmediate behave differently depending on context. Generally timers phase have priority over check phase, but in exceptional scenarios, especially inside I/O callbacks, check phase (setImmediate) will execute before timers phase because of how the event loop phases are structured.
5. Understand the event loop phases: Many developers mix up the order in which different types of tasks are processed by the event loop. Being clear on the order of the phases—timers, I/O callbacks, check, and close callbacks—will help you write more efficient code and answer interview questions confidently.

Bonus Question for You to Practice:

Here’s a slightly trickier question to practice:

console.log('Start');

setImmediate(() => {
  console.log('setImmediate');
});

Promise.resolve().then(() => {
  console.log('Promise 1');
}).then(() => {
  console.log('Promise 2');
});

setTimeout(() => {
  console.log('setTimeout');
}, 0);

console.log('End');        

What do you think the output will be? Try breaking it down step by step using the principles we’ve discussed, and see if you can predict the correct output.

Conclusion

Understanding how Node.js handles asynchronous code, especially the interaction between promises, setTimeout, setImmediate, and I/O callbacks, is critical to master JavaScript and Node.js interviews. Many interview questions center on how well you understand the event loop and how tasks are processed in different phases. The more comfortable you are with these concepts, the better equipped you’ll be to ace those tricky interview questions.

If you're preparing for a JavaScript interview, spend time experimenting with the event loop, reviewing Node.js documentation, and running different code examples to see the output in action.

Good luck with your prep, and feel free to reach out if you have any other questions! ??

References

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