Event loop overview in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the event loop in JavaScript?

The event loop is a process that allows JavaScript to perform non-blocking operations by handling asynchronous callbacks and managing the execution of code, events, and tasks in a single thread.

beginner

What are the main parts involved in the event loop?

The main parts are the call stack, the callback queue (or task queue), and the event loop itself which moves tasks from the queue to the stack when the stack is empty.

beginner

How does the call stack work in the event loop?

The call stack keeps track of function calls. When a function is called, it is added to the stack. When it finishes, it is removed. The event loop waits for the stack to be empty before processing new tasks.

beginner

What happens when an asynchronous operation completes in JavaScript?

When an async operation finishes, its callback is placed in the callback queue. The event loop then waits for the call stack to be empty before moving the callback to the stack to run.

beginner

Why is the event loop important for JavaScript?

Because JavaScript runs in a single thread, the event loop allows it to handle multiple tasks without freezing the program, making it responsive and efficient.

What does the event loop do in JavaScript?

A Stores variables and functions

B Runs multiple threads simultaneously

C Manages asynchronous callbacks and keeps the program responsive

D Compiles JavaScript code to machine code

The event loop manages asynchronous callbacks and ensures JavaScript stays responsive by handling tasks in a single thread.

Where are completed asynchronous callbacks placed before execution?

A Call stack

B Heap memory

C Global scope

D Callback queue

Completed asynchronous callbacks go to the callback queue, waiting for the call stack to be empty.

When does the event loop move a callback from the queue to the call stack?

A When the call stack is empty

B Immediately after the callback is created

C When the browser refreshes

D When the user clicks a button

The event loop moves callbacks to the call stack only when the stack is empty to avoid blocking.

Which of these is NOT part of the event loop mechanism?

A Database server

B Call stack

C Event loop

D Callback queue

The database server is external and not part of the event loop mechanism.

Why does JavaScript use an event loop?

A To run code faster by using multiple threads

B To handle asynchronous tasks without blocking the main thread

C To compile code before running

D To store user data

JavaScript uses the event loop to handle asynchronous tasks efficiently in a single thread.

Explain how the event loop manages asynchronous tasks in JavaScript.

Think about how JavaScript keeps running smoothly even with delays.

Describe the roles of the call stack and callback queue in the event loop.

Imagine a line of tasks waiting to be done one by one.

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