Angular Zone.js Error Fix

In this tutorial, you'll learn about Angular Zone.js Error Fix. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.

The Problem

Error: Zone.js has detected that Zone is already loaded.

Zone.js is loaded twice, typically from multiple script imports or incorrect Polyfill configuration.

Wrong

Including Zone.js in both polyfills.ts and <a href="/frameworks/angular/">angular</a>.json:

// angular.json
"scripts": ["node_modules/zone.js/dist/zone.js"]

While also importing it in polyfills.ts:

// polyfills.ts
import 'zone.js'

Output: Zone.js has detected that Zone is already loaded.

Right

Only import Zone.js in polyfills.ts (the default Angular setup):

// polyfills.ts
import 'zone.js'

Remove any duplicate script references from <a href="/frameworks/angular/">angular</a>.json:

// angular.json — remove Zone.js from scripts array
"scripts": []

Expected output: no Zone.js loading error.

Prevention

• Import Zone.js only in polyfills.ts
• Do not add Zone.js to <a href="/frameworks/angular/">Angular</a>.json scripts or index.html
• Use ng build and ng serve which handle polyfills automatically

Common Mistakes with zone js error

1. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks
2. Using return to exit a function early instead of wrapping a pure value in the monad
3. Mixing let bindings with <- bindings in do notation, producing type errors

These mistakes appear frequently in real-world Angular code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ