JavaScript Deep Dive Reading JavaScript Code You Didn’t Write

by

🟦 JavaScript Deep Dive — Issue #12

Reading JavaScript Code You Didn’t Write (And Understanding It Fast)

How senior engineers ramp up quickly in unfamiliar codebases

Writing JavaScript is one skill.
Reading and understanding existing JavaScript — especially someone else’s — is a completely different one.

Most developers struggle not because the code is “bad”…
but because they don’t have a system for understanding unfamiliar codebases.

Senior engineers do.

This issue teaches you how experienced developers read JavaScript code strategically, without getting lost or overwhelmed.

🧠 Why This Skill Matters More Than Writing Code

In real jobs, you spend:

  • 70–80% of your time reading code
  • 10–20% modifying it
  • Very little time writing from scratch

If you can’t understand existing code quickly:

  • You ship slower
  • You introduce bugs
  • You avoid refactors
  • You feel constantly behind

This is a career-defining skill.

🟨 1. Stop Reading Line-by-Line (That’s the Trap)

❌ Junior approach

“Let me start at the top and read every line.”

This leads to:

  • Cognitive overload
  • Lost context
  • Missed intent

✅ Senior approach

“What is this system trying to do?”

Understanding intent always comes before understanding implementation.

🟨 2. Start With the Entry Points

Every system has gateways.

Look for:

  • App initialization files
  • Routes
  • Event listeners
  • Public APIs
  • Exported functions

Ask:

  • What triggers this code?
  • Who calls this?
  • What problem does it solve?

Ignore the internals at first.

🟨 3. Identify the Shape of the System

Before details, find structure.

Ask:

  • Is this UI-driven, event-driven, or data-driven?
  • Is state centralized or scattered?
  • Is this layered, modular, or tightly coupled?

This gives you a mental map — without it, details are meaningless.

🟨 4. Follow Data, Not Files

New devs jump between files randomly.
Senior devs trace data flow.

Ask:

  • Where does data enter?
  • How is it transformed?
  • Where does it leave?

Example:

input → validation → transformation → side effect → output

Data flow reveals intent faster than syntax.

🟨 5. Ignore Clever Code (At First)

Clever abstractions are tempting — and distracting.

Don’t start with:

  • Utility helpers
  • Generic libraries
  • Meta-programming
  • Custom hooks
  • Shared utils

Start with boring code:

  • Handlers
  • Controllers
  • Services
  • Business logic

That’s where meaning lives.

🟨 6. Decode Naming, Not Syntax

Names tell you more than code.

Pay attention to:

  • Function names
  • Variable names
  • Folder names
  • File boundaries

Bad names = harder system
Good names = easier mental model

If names are vague, write better ones in your head as you read.

🟨 7. Use the Code to Answer Questions (Not to Memorize)

Instead of:

“What does this code do?”

Ask:

  • What inputs does this expect?
  • What assumptions does it make?
  • What happens if this fails?
  • What state does this depend on?

Senior devs interrogate code like a witness.

🟨 8. Debug to Understand Faster

You don’t need full understanding to run code.

Tactics:
✔ Add breakpoints
✔ Log critical values
✔ Trigger edge cases
✔ Watch state change
✔ Use the call stack

Running code collapses uncertainty fast.

🟨 9. Spot Architecture Smells Early

These slow understanding immediately:

🚩 God files
🚩 Hidden side effects
🚩 Global mutable state
🚩 Tight coupling
🚩 Circular dependencies
🚩 “Magic” utilities

Spotting these helps you adjust expectations and avoid wrong assumptions.

🟨 10. Build a Mental “Summary” of the System

Senior engineers constantly update a simple mental summary:

“This app does X.
Data flows from A → B → C.
State lives here.
Side effects happen there.”

You don’t need perfection — just enough clarity to move safely.

🧩 Mini Exercises

1. What’s the first file you’d open?

/handlers
/services
/utils
/index.js

2. What question would you ask first?

export function processOrder(order) {
  if (!order) return;
  save(order);
}

3. Why is this harder to understand than it looks?

doThing(doOtherThing(getValue()));

🟦 Senior-Level Reading Checklist

✔ Find entry points
✔ Understand intent first
✔ Trace data, not files
✔ Ignore clever code initially
✔ Decode naming
✔ Ask “why” constantly
✔ Run the code early
✔ Maintain a mental summary

🏁 Final Thoughts

Great developers aren’t fast typists.
They’re fast understanders.

If you can:

  • Read unfamiliar JavaScript confidently
  • Build mental models quickly
  • Modify code safely

You become invaluable — regardless of framework or stack.