Event loop overview in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to log 'Hello' after 1 second using setTimeout.

Javascript

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

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

A0

B1

C500

D1000

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of 1000 causes almost no delay.

Using 0 runs immediately after current code.

The setTimeout function delays execution by the given milliseconds. 1000 ms equals 1 second.

2fill in blank

medium

Complete the code to add a function to the event loop queue using Promise.resolve().then.

Javascript

Promise.resolve().[1](() => console.log('Done'));

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

Athen

Bcatch

Cfinally

DsetTimeout

Attempts:

3 left

💡 Hint

Common Mistakes

Using catch instead of then.

Using setTimeout which schedules a macrotask.

The then method schedules the callback to run after the current call stack, using the microtask queue.

3fill in blank

hard

Fix the error in the code to correctly log 'First' then 'Second' using setTimeout.

Javascript

console.log('First');
setTimeout(() => console.log('Second'), [1]);

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

A1000

B0

C'0'

D'1000'

Attempts:

3 left

💡 Hint

Common Mistakes

Using string '1000' instead of number 1000.

Using 0 or '0' which causes immediate callback.

The delay must be a number, not a string. 1000 means 1 second delay.

4fill in blank

hard

Fill both blanks to create a microtask that logs 'Microtask' and a macrotask that logs 'Macrotask'.

Javascript

Promise.resolve().[1](() => console.log('Microtask'));
setTimeout(() => console.log('Macrotask'), [2]);

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

Athen

Bcatch

C0

D1000

Attempts:

3 left

💡 Hint

Common Mistakes

Using catch instead of then.

Using 1000 delay which delays macrotask by 1 second.

then schedules a microtask, which runs before macrotasks like setTimeout. Delay 0 means run as soon as possible after current tasks.

5fill in blank

hard

Fill all three blanks to create a code that logs 'Start', then a microtask 'Middle', then a macrotask 'End'.

Javascript

console.log([1]);
Promise.resolve().[2](() => console.log('Middle'));
setTimeout(() => console.log([3]), 0);

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

A'Start'

Bthen

C'End'

Dcatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using catch instead of then.

Swapping 'Start' and 'End' logs.

Logging 'Start' first, then scheduling a microtask with then to log 'Middle', and a macrotask with setTimeout to log 'End'. This shows event loop order.

Practice

1. What is the main role of the JavaScript event loop?

easy

A. To manage execution of multiple tasks without stopping the program

B. To compile JavaScript code into machine code

C. To store variables and functions in memory

D. To style HTML elements dynamically

Solution

1. Step 1: Understand event loop purpose

   The event loop allows JavaScript to handle many tasks smoothly by managing task execution order.

2. Step 2: Compare options with event loop role

   Only To manage execution of multiple tasks without stopping the program describes managing multiple tasks without stopping, which matches the event loop's job.

3. Final Answer:

   To manage execution of multiple tasks without stopping the program -> Option A

4. Quick Check:

   Event loop = manages tasks smoothly [OK]

Hint: Event loop runs tasks one by one without freezing [OK]

Common Mistakes:

• Confusing event loop with code compilation
• Thinking event loop handles styling
• Believing event loop stores variables

2. Which of the following is the correct syntax to schedule a function to run after the current code using the event loop?

easy

A. setTimeout(myFunction, 0);

B. runLater(myFunction);

C. delay(myFunction, 0);

D. executeAfter(myFunction);

Solution

1. Step 1: Identify standard JavaScript function for scheduling

   setTimeout is the built-in function to schedule code after current execution.

2. Step 2: Check options for correct syntax

   Only setTimeout(myFunction, 0) is valid JavaScript syntax to schedule a function immediately after current code.

3. Final Answer:

   setTimeout(myFunction, 0); -> Option A

4. Quick Check:

   Use setTimeout(fn, 0) to schedule tasks [OK]

Hint: Remember setTimeout(fn, 0) delays after current code [OK]

Common Mistakes:

• Using non-existent functions like runLater or delay
• Omitting the delay argument in setTimeout
• Confusing setTimeout with synchronous calls

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

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

medium

A. StartTimeoutEnd

B. StartEndTimeout

C. TimeoutStartEnd

D. EndStartTimeout

Solution

1. Step 1: Understand synchronous and asynchronous order

   console.log('Start') runs first, then setTimeout schedules 'Timeout' to run later, then console.log('End') runs immediately.

2. Step 2: Determine output order

   'Start' prints, then 'End' prints, then after current code finishes, 'Timeout' prints from the event loop.

3. Final Answer:

   Start
   End
   Timeout -> Option B

4. Quick Check:

   Sync logs first, then setTimeout logs [OK]

Hint: setTimeout runs after current code finishes [OK]

Common Mistakes:

• Assuming setTimeout runs immediately
• Mixing order of synchronous logs
• Ignoring event loop delay

4. Identify the error in this code snippet related to the event loop:

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

medium

A. setTimeout requires three arguments

B. Arrow functions cannot be used with setTimeout

C. Missing parentheses after console.log

D. The delay should be a number, not a string

Solution

1. Step 1: Check setTimeout delay argument type

   The delay argument must be a number, but here it is a string '1000'.

2. Step 2: Understand effect of wrong type

   Passing a string may cause unexpected behavior or be coerced, but best practice is to use a number.

3. Final Answer:

   The delay should be a number, not a string -> Option D

4. Quick Check:

   Delay must be number in setTimeout [OK]

Hint: Use number for delay in setTimeout, not string [OK]

Common Mistakes:

• Using string instead of number for delay
• Thinking arrow functions are invalid here
• Assuming setTimeout needs 3 arguments

5. You want to run two functions, first() and second(), so that second() runs only after first() finishes, but first() contains asynchronous code using setTimeout. How can you ensure second() runs after first() completes all its tasks?

hard

A. Use two separate setTimeout calls with the same delay

B. Call second() immediately after calling first()

C. Call second() inside the callback of setTimeout in first()

D. Use a loop to wait until first() finishes

Solution

1. Step 1: Understand asynchronous behavior of setTimeout

   setTimeout runs code later, so code after first() runs immediately without waiting.

2. Step 2: Ensure second() runs after first()'s async code

   To guarantee order, call second() inside the setTimeout callback in first(), so it runs after the async task.

3. Final Answer:

   Call second() inside the callback of setTimeout in first() -> Option C

4. Quick Check:

   Chain async calls inside callbacks to keep order [OK]

Hint: Put second() inside first()'s async callback [OK]

Common Mistakes:

• Calling second() immediately after first() ignoring async
• Using multiple setTimeouts with same delay expecting order
• Trying to block with loops instead of callbacks