JavaScript Fundamentals

Task Queue

How JavaScript queues and processes asynchronous callbacks

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What are Task Queues?

Task queues are waiting lines for callbacks. JavaScript has two types: Microtask Queue (high priority) and Macrotask Queue (low priority)!

Priority System

The event loop always checks: ALL microtasks first, then ONE macrotask, then repeat. Microtasks cut in line!

Task Queue Animation

Watch how tasks are queued and processed

Microtask Queue

HIGH PRIORITY

Empty

Promises, queueMicrotask

Macrotask Queue

LOW PRIORITY

Empty

setTimeout, setInterval, I/O

Currently Processing

Nothing executing

Console Output

// No output yet

Step 1 of 14

Program starts - both queues empty

Code Executing

console.log('Start');

Queue Processing Rules

How the event loop processes tasks

Execute Synchronous Code

All synchronous code runs first, immediately

Process ALL Microtasks

Empty the entire microtask queue before moving on

Process ONE Macrotask

Execute only one macrotask, then go back to step 2

♻️

Repeat

Loop forever: check microtasks → process one macrotask → repeat

Task Queue Example

Visualizing queue priority

javascript

code

DarkEditable

Console Output

Click "Run Code" to see output here...

Microtasks Can Starve Macrotasks

Be careful with infinite microtasks!

javascript

code

DarkEditable

Console Output

Click "Run Code" to see output here...

Key Takeaways

Remember ✅

• Two queues: Microtask & Macrotask
• Microtasks have higher priority
• ALL microtasks run before next macrotask
• Only ONE macrotask per cycle
• Event loop checks microtasks after each task

Watch Out ❌

• Don't create infinite microtask loops
• Don't assume setTimeout runs immediately
• Don't mix sync/async assumptions
• Don't block the event loop
• Don't forget promises are microtasks

Quick Reference

Microtasks: Promises, queueMicrotask(), async/await

Macrotasks: setTimeout, setInterval, I/O, UI events

Priority: Sync → All Micros → One Macro → Repeat

Rule: Microtasks can't be interrupted by macrotasks

Pro Tip

Use queueMicrotask() when you need something to run as a microtask but don't want to use promises!

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Event Loop

Understanding how JavaScript handles asynchronous operations and concurrency.

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Understanding the Document Object Model and its tree structure.

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Output

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JavaScript Fundamentals

Task Queue

How JavaScript queues and processes asynchronous callbacks

Find videos you like?

Save to resource drawer for future reference!

What are Task Queues?

Task queues are waiting lines for callbacks. JavaScript has two types: Microtask Queue (high priority) and Macrotask Queue (low priority)!

Priority System

The event loop always checks: ALL microtasks first, then ONE macrotask, then repeat. Microtasks cut in line!

Task Queue Animation

Watch how tasks are queued and processed

Microtask Queue

HIGH PRIORITY

Empty

Promises, queueMicrotask

Macrotask Queue

LOW PRIORITY

Empty

setTimeout, setInterval, I/O

Currently Processing

Nothing executing

Console Output

// No output yet

Step 1 of 14

Program starts - both queues empty

Code Executing

console.log('Start');

Queue Processing Rules

How the event loop processes tasks

Execute Synchronous Code

All synchronous code runs first, immediately

Process ALL Microtasks

Empty the entire microtask queue before moving on

Process ONE Macrotask

Execute only one macrotask, then go back to step 2

♻️

Repeat

Loop forever: check microtasks → process one macrotask → repeat

Task Queue Example

Visualizing queue priority

javascript

code

DarkEditable

Console Output

Click "Run Code" to see output here...

Microtasks Can Starve Macrotasks

Be careful with infinite microtasks!

javascript

code

DarkEditable

Console Output

Click "Run Code" to see output here...

Key Takeaways

Remember ✅

• Two queues: Microtask & Macrotask
• Microtasks have higher priority
• ALL microtasks run before next macrotask
• Only ONE macrotask per cycle
• Event loop checks microtasks after each task

Watch Out ❌

• Don't create infinite microtask loops
• Don't assume setTimeout runs immediately
• Don't mix sync/async assumptions
• Don't block the event loop
• Don't forget promises are microtasks

Quick Reference

Microtasks: Promises, queueMicrotask(), async/await

Macrotasks: setTimeout, setInterval, I/O, UI events

Priority: Sync → All Micros → One Macro → Repeat

Rule: Microtasks can't be interrupted by macrotasks

Pro Tip

Use queueMicrotask() when you need something to run as a microtask but don't want to use promises!

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Event Loop

What is DOM?

JavaScript Code Editor

Building Your Learning Module...

Task Queue

What are Task Queues?

Priority System

Microtask Queue

Macrotask Queue

Currently Processing

Console Output

Execute Synchronous Code

Process ALL Microtasks

Process ONE Macrotask

Repeat

Task Queue Example

Microtasks Can Starve Macrotasks

Key Takeaways

Remember ✅

Watch Out ❌

Quick Reference

Pro Tip

Event Loop

What is DOM?

JavaScript Code Editor

Task Queue

What are Task Queues?

Priority System

Microtask Queue

Macrotask Queue

Currently Processing

Console Output

Execute Synchronous Code

Process ALL Microtasks

Process ONE Macrotask

Repeat

Task Queue Example

Microtasks Can Starve Macrotasks

Key Takeaways

Remember ✅

Watch Out ❌

Quick Reference

Pro Tip