Event loop mental model in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to schedule a function to run after the current event loop phase.

Node.js

setTimeout(() => { console.log('Hello'); }, [1]);

Drag options to blanks, or click blank then click option'

A0

B1000

Cnull

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using negative or null values for delay causes errors or unexpected behavior.

Using 0 as the delay schedules the function to run after the current event loop phase.

2fill in blank

medium

Complete the code to add a callback to the next tick queue.

Node.js

process.[1](() => { console.log('Next tick'); });

Drag options to blanks, or click blank then click option'

AnextTick

BsetInterval

CsetTimeout

DsetImmediate

Attempts:

3 left

💡 Hint

Common Mistakes

Using setImmediate or setTimeout delays the callback to later phases.

process.nextTick schedules a callback to run immediately after the current operation, before other event loop phases.

3fill in blank

hard

Fix the error in the code to correctly schedule a callback after the current timers phase.

Node.js

setImmediate(() => { console.log('Immediate'); });
setTimeout(() => { console.log('Timeout'); }, [1]);

Drag options to blanks, or click blank then click option'

A-1

B0

Cundefined

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using null or negative values for timeout delay.

Using 0 as the delay in setTimeout schedules the callback after the current timers phase, matching expected behavior.

4fill in blank

hard

Fill both blanks to create a map of timers with delays and their callback names.

Node.js

const timers = { [1]: 'first', [2]: 'second' };

Drag options to blanks, or click blank then click option'

A100

BsetTimeout

C200

DsetImmediate

Attempts:

3 left

💡 Hint

Common Mistakes

Using function names as keys instead of numbers.

The keys are timer delays in milliseconds, so 100 and 200 are correct.

5fill in blank

hard

Fill all three blanks to create a filtered object of callbacks with delays greater than 100.

Node.js

const filtered = Object.entries(timers).reduce((acc, [[1], [2]]) => {
  if ([1] [3] 100) {
    acc[[1]] = [2];
  }
  return acc;
}, {});

Drag options to blanks, or click blank then click option'

Adelay

Bcb

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names or wrong comparison operators.

The reduce uses delay as key, cb as value, and filters delays greater than 100.

Practice

1. Which part of the Node.js event loop runs Promise callbacks before timers?

easy

A. I/O callbacks phase

B. Timers phase

C. Microtasks queue

D. Check phase

Solution

1. Step 1: Understand event loop phases

   The event loop has phases: timers, I/O callbacks, idle, poll, check, close callbacks, and microtasks run between phases.

2. Step 2: Identify when Promise callbacks run

   Promise callbacks are microtasks and run immediately after the current operation, before timers and I/O callbacks.

3. Final Answer:

   Microtasks queue -> Option C

4. Quick Check:

   Promises run in microtasks before timers [OK]

Hint: Remember: promises run before timers in microtasks [OK]

Common Mistakes:

• Thinking timers run before promises
• Confusing I/O callbacks with microtasks
• Assuming check phase runs before microtasks

2. Which of the following is the correct syntax to schedule a function to run after 0 milliseconds in Node.js?

easy

A. setTimeout(myFunc, 0);

B. setInterval(myFunc, 0);

C. process.nextTick(myFunc);

D. setImmediate(myFunc, 0);

Solution

1. Step 1: Identify function to run after delay

   setTimeout schedules a function after a specified delay in milliseconds.

2. Step 2: Check syntax for zero delay

   Using setTimeout(myFunc, 0) runs myFunc after the current call stack is empty, effectively scheduling it soon.

3. Final Answer:

   setTimeout(myFunc, 0); -> Option A

4. Quick Check:

   setTimeout with 0 delay schedules function correctly [OK]

Hint: Use setTimeout(func, 0) to schedule next tick [OK]

Common Mistakes:

• Using setInterval for one-time delay
• Passing extra argument to setImmediate
• Confusing process.nextTick with setTimeout syntax

3. What will be the output order of the following code?

console.log('start');
setTimeout(() => console.log('timeout'), 0);
Promise.resolve().then(() => console.log('promise'));
console.log('end');

medium

A. promisestartendtimeout

B. startpromiseendtimeout

C. startendtimeoutpromise

D. startendpromisetimeout

Solution

1. Step 1: Identify synchronous and asynchronous parts

   console.log('start') and console.log('end') run immediately (synchronously). setTimeout callback runs later. Promise callback runs as microtask after current stack.

2. Step 2: Trace execution order

   Output order: 'start' (sync), 'end' (sync), 'promise' (microtask), 'timeout' (timer callback).

3. Final Answer:

   start
   end
   promise
   timeout -> Option D

4. Quick Check:

   Synchronous > microtasks > timers [OK]

Hint: Sync logs first, then promises, then timers [OK]

Common Mistakes:

• Thinking promise runs after timeout
• Mixing order of synchronous logs
• Assuming setTimeout runs immediately

4. Consider this code snippet:

setTimeout(() => console.log('timeout'));
process.nextTick(() => console.log('nextTick'));
Promise.resolve().then(() => console.log('promise'));

Which line causes the earliest callback to run, and why might the output order be unexpected?

medium

A. process.nextTick runs earliest because it runs before microtasks

B. setTimeout runs earliest because timers run first

C. Promise.then runs earliest because promises run before nextTick

D. All callbacks run simultaneously

Solution

1. Step 1: Understand callback priorities

   process.nextTick callbacks run immediately after the current operation, before promise microtasks and timers.

2. Step 2: Explain output order

   Even though promises are microtasks, process.nextTick callbacks have higher priority and run first, which can surprise learners expecting promises first.

3. Final Answer:

   process.nextTick runs earliest because it runs before microtasks -> Option A

4. Quick Check:

   nextTick > promises > timers [OK]

Hint: nextTick runs before promises and timers [OK]

Common Mistakes:

• Assuming timers run before nextTick
• Confusing promise and nextTick order
• Thinking callbacks run simultaneously

5. You want to run a CPU-heavy task without blocking the event loop in Node.js. Which approach best uses the event loop model to keep your app responsive?

hard

A. Run the task synchronously in the main thread

B. Use setTimeout to split the task into smaller chunks

C. Use process.nextTick to run the entire task immediately

D. Run the task inside a Promise without splitting

Solution

1. Step 1: Understand event loop blocking

   Running a heavy task synchronously blocks the event loop, making the app unresponsive.

2. Step 2: Choose non-blocking approach

   Splitting the task into smaller chunks with setTimeout allows the event loop to process other events between chunks, keeping responsiveness.

3. Final Answer:

   Use setTimeout to split the task into smaller chunks -> Option B

4. Quick Check:

   Split heavy tasks with timers to avoid blocking [OK]

Hint: Split heavy tasks with setTimeout to avoid blocking [OK]

Common Mistakes:

• Running heavy tasks synchronously
• Using nextTick for long tasks (blocks event loop)
• Assuming promises alone prevent blocking