Event loop phases and timer execution in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to schedule a function to run after 1000 milliseconds.

Node.js

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

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

Anull

B10

C0

D1000

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or null will cause the callback to run immediately or not at all.

Confusing seconds with milliseconds.

The setTimeout function schedules the callback to run after the specified milliseconds. Here, 1000 means 1 second delay.

2fill in blank

medium

Complete the code to schedule a function to run repeatedly every 500 milliseconds.

Node.js

const intervalId = setInterval(() => { console.log('Repeating every 0.5 seconds'); }, [1]);

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

A500

B1000

C50

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 will cause the callback to run as fast as possible, which is not intended.

Confusing interval time with timeout delay.

The setInterval function runs the callback repeatedly every given milliseconds. 500 means every half second.

3fill in blank

hard

Fix the error in the code to clear the interval after 3 seconds.

Node.js

const id = setInterval(() => { console.log('Tick'); }, 1000);
setTimeout(() => { clearInterval([1]); }, 3000);

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

AtimerId

BintervalId

Cid

DtimeoutId

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that was never declared.

Confusing interval ID with timeout ID.

The setInterval returns an ID which must be passed to clearInterval to stop it. Here, the ID is stored in id.

4fill in blank

hard

Fill both blanks to create a Promise that resolves after 2 seconds using setTimeout.

Node.js

const delay = ms => new Promise(resolve => {
  setTimeout(() => {
    [1]();
  }, [2]);
});

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

Aresolve

Bms

Cconsole.log

DclearTimeout

Attempts:

3 left

💡 Hint

Common Mistakes

Calling reject instead of resolve.

Using console.log instead of resolve.

Passing wrong delay time.

Inside the Promise, resolve is called to mark it as done. The delay time is passed as milliseconds to setTimeout.

5fill in blank

hard

Fill all three blanks to create a map of words to their lengths, but only for words longer than 3 characters.

Node.js

const words = ['apple', 'bat', 'carrot', 'dog'];
const lengths = words.reduce((acc, [3]) => {
  if ([3].length > 3) {
    acc[[1]] = [2];
  }
  return acc;
}, {});

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

Aword

Bword.length

Ditem

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names.

Using length as key instead of word.

Not filtering words by length.

The reduce method creates an object that maps each word to its length word.length for words longer than 3 characters.

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