While developing an SPFx Application Customizer, I encountered a confusing runtime error:

❌ Cannot read properties of undefined (reading 'id')

What made this issue especially difficult to debug was that:

• The code compiled without errors

• The issue appeared only when another component was present on the page

• The failure happened at runtime, not during build

After extensive debugging, the root cause turned out to be a React version incompatibility.

❓ The Actual Problem

SPFx tightly controls which React version it supports.

In this case:

• SPFx required: react@17.0.1

• Installed version: react@17.0.2

Although this looks like a harmless patch upgrade, SPFx requires an exact React version match.

This mismatch caused:

• Multiple React instances to load

• Unstable rendering behavior

• Runtime errors such as:

Cannot read properties of undefined (reading 'id')

🧠 Why This Error Happens

SPFx:

• Treats React as an external, preloaded dependency

• Assumes a single, exact React version

• Breaks silently when that assumption is violated

When react@17.0.2 was installed:

• Hooks and lifecycle logic behaved inconsistently

• Context objects became undefined

• Application Customizers failed unpredictably

The error surfaced only when another component was present because:

• The second component triggered additional renders

• SPA navigation exposed the duplicate React runtime

🔍 How I Diagnosed the Issue

1️⃣ Checked installed React versions

npm ls react react-dom

Output showed:

react@17.0.2
react-dom@17.0.2

But SPFx expected:

react@17.0.1
react-dom@17.0.1

2️⃣ Confirmed duplicate React renderers

Using browser DevTools:

__REACT_DEVTOOLS_GLOBAL_HOOK__

This revealed multiple React renderers, confirming a version conflict.

✅ The Fix

Step 1: Install the exact SPFx-supported versions

npm install react@17.0.1 react-dom@17.0.1 --save-exact

Step 2: Clean the environment

rm -rf node_modules package-lock.json
npm install

Step 3: Verify

npm ls react react-dom

✔ Both resolved to 17.0.1

🎉 Result

• The error “Cannot read properties of undefined (reading 'id')” disappeared

• Application Customizer rendered consistently

• No failures when other components were present

• Stable behavior across SPA navigation

⚠️ Key Takeaways

• SPFx requires exact React version alignment

• Patch-level differences (17.0.1 vs 17.0.2) matter

• Runtime errors may look unrelated to React

• Application Customizers are the first to break

📝 Best Practices for SPFx Projects

npm install react@17.0.1 react-dom@17.0.1 --save-exact

✔ Match SPFx documentation

✔ Avoid ^ or ~ in version ranges

✔ Re-check versions after installing new packages

📌 Conclusion

If you encounter the error:

Cannot read properties of undefined (reading 'id')

in an SPFx Application Customizer, especially when:

• Another component is on the page

• There are no build-time errors

👉 Check your React version first.

Sometimes the bug isn’t your code — it’s a 0.0.1 React mismatch.