Async Programming & Performance in JavaScript – What Actually Happens Behind the Scenes

Who Am I?

Nirmal Vyas

18+ years experience in IT field

Worked almost 10+ years in various IT companies as a Developer, Senior developer, Team lead, Project Manager etc.

Fulltime Freelancer/Consultant since last 8 + years
I do Web development projects for various clients in India and abroad, and also Technical Consultancy Services for various IT companies and startups.
 

Website: http://codenirmal.com/

Organizer of Technical meetups in Kochi like Kerala JS, Active Member of other groups like Kochi Ruby, Kochi Python etc.

Introduction

Content:

• JS is single-threaded — what does that really mean?
• Why async programming exists
• Common confusion:
  • async/await ≠ parallel
  • setTimeout ≠ immediate
  • “Node.js is fast” — why?

Trigger Questions:

• “Why doesn’t Node crash with 1000 users?”
• “Why is my API slow even though code is correct?”

JavaScript Execution Model

Content:

• Call Stack (what runs now)
• Single-threaded nature
• Blocking vs Non-blocking

Key Takeaway:

• Only one thing runs at a time

Event Loop Deep Dive

Content:

• What is Event Loop?
• Web APIs / Node APIs
• Callback Queue (macrotask queue)
• Microtask Queue (Promises)

Flow:

Code → Call Stack
Async → API
Done → Queue
Event Loop → Execute

4. Promises & async/await
 

Content:

• Promise basics (resolve/reject)
• async/await = syntactic sugar
• Execution flow behind await
   

Key Misconceptions:

• await does NOT make code parallel
• await blocks function execution

Sequential vs Parallel Execution 

Content:

• Sequential execution (slow)
• Parallel execution with Promise.all
   

Key Misconceptions:

• await does NOT make code parallel
• await blocks function execution

Common Performance Mistakes

Content:

• ❌ Blocking Code

• Infinite loops
• Heavy sync operations

• ❌ Misuse of await

• Inside loops

• ❌ Large JSON parsing

• CPU blocking

• ❌ Too many sequential API calls

Fix Patterns​:

• Promise.all Batching
• Offloading work

Backend Performance Thinking

Content:

• API response lifecycle
• DB + API + File system → async flow
• Identifying bottlenecks

Key Idea:

•  Performance = execution order + resource usage

 Debugging Async Issues

Content:

• How to identify slow parts

• Logs + timing

• Where things usually fail

Summary & Key Takeaways

• JS is single-threaded but non-blocking

• Event loop controls everything

• async/await is not magic

• Performance depends on how you structure async code

Questions?

nirmalkamath@gmail.com
Whatsapp: 9895625954

Thanks!

nirmalkamath@gmail.com

http://codenirmal.com