Event loop phases and timer execution in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding setTimeout and setImmediate order

Consider this Node.js code snippet:

setTimeout(() => console.log('timeout'), 0);
setImmediate(() => console.log('immediate'));

What is the most likely output order when this code runs?

Node.js

setTimeout(() => console.log('timeout'), 0);
setImmediate(() => console.log('immediate'));

A timeout only

B immediate\ntimeout

C timeout\nimmediate

D immediate only

💡 Hint

Think about which phase runs first after the current poll phase.

In Node.js, setTimeout callbacks with 0 delay run in the timers phase, which happens before the check phase where setImmediate callbacks run. So timeout logs before immediate.

❓ component_behavior

intermediate

Behavior of process.nextTick in event loop

What happens when you schedule a callback with process.nextTick inside a setTimeout callback?

setTimeout(() => {
  console.log('timeout');
  process.nextTick(() => console.log('nextTick inside timeout'));
}, 0);

What will be the output order?

Node.js

setTimeout(() => {
  console.log('timeout');
  process.nextTick(() => console.log('nextTick inside timeout'));
}, 0);

A timeout\nnextTick inside timeout

B nextTick inside timeout\ntimeout

C Only timeout

D Only nextTick inside timeout

💡 Hint

Remember when nextTick callbacks run relative to the current phase.

process.nextTick callbacks run immediately after the current operation completes, before the event loop continues. So inside the setTimeout callback, timeout logs first, then the nextTick callback runs.

📝 Syntax

advanced

Identify error in timer callback usage

Which option will cause a runtime error when executed in Node.js?

setTimeout(() => console.log('Hello'), '1000');

Node.js

setTimeout(() => console.log('Hello'), '1000');

A ReferenceError because setTimeout is undefined

B TypeError because delay must be a number, not string

C SyntaxError due to arrow function

D No error, prints 'Hello' after 1 second

💡 Hint

Check how Node.js treats string delays in setTimeout.

Node.js converts string delays to numbers automatically, so no error occurs. The callback runs after approximately 1 second.

🔧 Debug

advanced

Why does this setInterval never stop?

Consider this code:

let count = 0;
setInterval(() => {
  if (count === 3) clearInterval();
  console.log(count);
  count++;
}, 1000);

Why does the interval never stop?

Node.js

let count = 0;
const intervalId = setInterval(() => {
  if (count === 3) clearInterval(intervalId);
  console.log(count);
  count++;
}, 1000);

A clearInterval is called without interval ID, so it does nothing

B count never reaches 3 because of async behavior

C Syntax error due to missing semicolon

D setInterval is not cleared because count is reset

💡 Hint

What does clearInterval need to stop an interval?

clearInterval requires the interval ID returned by setInterval. Calling it without arguments does nothing, so the interval keeps running.

❓ lifecycle

expert

Order of phases with nested timers and nextTick

What is the output order of this Node.js code?

console.log('start');
setTimeout(() => {
  console.log('timeout 1');
  process.nextTick(() => console.log('nextTick inside timeout 1'));
}, 0);
setImmediate(() => {
  console.log('immediate 1');
  setTimeout(() => console.log('timeout 2'), 0);
});
process.nextTick(() => console.log('nextTick 1'));
console.log('end');

Node.js

console.log('start');
setTimeout(() => {
  console.log('timeout 1');
  process.nextTick(() => console.log('nextTick inside timeout 1'));
}, 0);
setImmediate(() => {
  console.log('immediate 1');
  setTimeout(() => console.log('timeout 2'), 0);
});
process.nextTick(() => console.log('nextTick 1'));
console.log('end');

A start\nend\nnextTick 1\ntimeout 1\nnextTick inside timeout 1\nimmediate 1\ntimeout 2

B start\nend\nnextTick 1\nimmediate 1\ntimeout 1\nnextTick inside timeout 1\ntimeout 2

C start\nend\nimmediate 1\nnextTick 1\ntimeout 1\nnextTick inside timeout 1\ntimeout 2

D start\nend\nnextTick 1\ntimeout 1\nimmediate 1\nnextTick inside timeout 1\ntimeout 2

💡 Hint

Remember process.nextTick runs immediately after current operation, and setImmediate runs after poll phase.

Execution order:
1. 'start' and 'end' log immediately.
2. After main script completes (end of poll phase), process.nextTick callbacks run: 'nextTick 1' logs.
3. Check phase runs: 'immediate 1' logs, schedules 'timeout 2'.
4. Timers phase (next iteration): 'timeout 1' logs.
5. After 'timeout 1' callback, process.nextTick runs: 'nextTick inside timeout 1' logs.
6. Next timers phase: 'timeout 2' logs.

Practice

1. Which phase of the Node.js event loop executes setTimeout callbacks?

easy

A. Check phase

B. Timers phase

C. Poll phase

D. Close callbacks phase

Solution

1. Step 1: Understand event loop phases

   The Node.js event loop has multiple phases, each handling different types of callbacks.

2. Step 2: Identify where timers run

   The timers phase is specifically designed to execute callbacks scheduled by setTimeout and setInterval.

3. Final Answer:

   Timers phase -> Option B

4. Quick Check:

   Timers phase = setTimeout callbacks [OK]

Hint: Timers run in the timers phase, not immediately [OK]

Common Mistakes:

• Confusing timers phase with poll phase
• Thinking setTimeout runs immediately
• Mixing check phase with timers phase

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

easy

A. setTimeout(console.log('Hi'))

B. setTimeout(console.log('Hi'), 0);

C. setTimeout(0, () => console.log('Hi'));

D. setTimeout(() => console.log('Hi'), 0);

Solution

1. Step 1: Check function syntax for setTimeout

   The first argument must be a function, not the result of a function call.

2. Step 2: Analyze each option

   setTimeout(() => console.log('Hi'), 0); passes a function that logs 'Hi' after 0 ms delay correctly. setTimeout(console.log('Hi'), 0); calls console.log immediately and passes its result (undefined). setTimeout(0, () => console.log('Hi')); passes 0 (number) as first argument instead of a function, causing a TypeError on execution. setTimeout(console.log('Hi')) calls console.log immediately without delay argument.

3. Final Answer:

   setTimeout(() => console.log('Hi'), 0); -> Option D

4. Quick Check:

   Function as first argument = setTimeout(() => console.log('Hi'), 0); [OK]

Hint: Pass a function, not a function call, to setTimeout [OK]

Common Mistakes:

• Calling the function immediately inside setTimeout
• Omitting the delay argument
• Passing non-function as first argument

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

console.log('Start');
setTimeout(() => console.log('Timeout 1'), 0);
setTimeout(() => console.log('Timeout 2'), 10);
console.log('End');

medium

A. Start, End, Timeout 1, Timeout 2

B. Start, Timeout 1, Timeout 2, End

C. Timeout 1, Timeout 2, Start, End

D. Start, Timeout 2, End, Timeout 1

Solution

1. Step 1: Identify synchronous and asynchronous parts

   console.log('Start') and console.log('End') run immediately in order. setTimeout callbacks run later.

2. Step 2: Understand timer delays and event loop

   setTimeout with 0 ms delay runs after current code finishes, before 10 ms delay callback.

3. Final Answer:

   Start, End, Timeout 1, Timeout 2 -> Option A

4. Quick Check:

   Synchronous logs first, then timers by delay [OK]

Hint: Synchronous logs run before any setTimeout callbacks [OK]

Common Mistakes:

• Assuming 0 ms timeout runs immediately
• Mixing order of synchronous and asynchronous logs
• Ignoring timer delays

4. Identify the error in this code snippet:

setTimeout(console.log('Hello'), 1000);

medium

A. The delay argument is missing

B. The delay must be 0 or less

C. console.log is called immediately instead of after 1000ms

D. setTimeout requires a string as first argument

Solution

1. Step 1: Analyze the first argument of setTimeout

   console.log('Hello') is called immediately, returning undefined, which is passed to setTimeout.

2. Step 2: Understand correct usage

   setTimeout expects a function as first argument, not the result of a function call.

3. Final Answer:

   console.log is called immediately instead of after 1000ms -> Option C

4. Quick Check:

   Function call inside setTimeout runs immediately [OK]

Hint: Pass a function, not a function call, to setTimeout [OK]

Common Mistakes:

• Thinking delay argument is missing
• Passing string instead of function
• Believing delay must be zero or negative

5. Consider this code:

console.log('A');
setTimeout(() => console.log('B'), 0);
Promise.resolve().then(() => console.log('C'));
console.log('D');

What is the correct order of output?

hard

A. A, D, C, B

B. A, B, D, C

C. A, D, B, C

D. A, C, D, B

Solution

1. Step 1: Identify synchronous, microtask, and timers

   console.log('A') and console.log('D') run immediately. Promise.then callbacks run in microtasks after current code. setTimeout callbacks run in timers phase later.

2. Step 2: Determine execution order

   Output order is synchronous logs first (A, D), then microtasks (C), then timers (B).

3. Final Answer:

   A, D, C, B -> Option A

4. Quick Check:

   Microtasks run before timers [OK]

Hint: Promise.then runs before setTimeout even with 0 delay [OK]

Common Mistakes:

• Assuming setTimeout runs before Promise.then
• Mixing synchronous and asynchronous order
• Ignoring microtask queue priority