Troubleshooting

How to use this section

Every entry follows the same structure so you can scan it quickly during an incident:

1. Symptom — the exact message or observable behaviour.
2. Cause — what Angular is actually complaining about.
3. Diagnosis — how to confirm it in your own code.
4. Fix — the concrete change.
5. Prevention — how to stop it from coming back.

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Quick error index

Code	Message	Page
NG0100	ExpressionChangedAfterItHasBeenCheckedError	Runtime errors
NG0200	Circular dependency in DI	DI errors
NG0201	No provider for X	DI errors
NG0203	inject() must be called from an injection context	DI errors
NG0301	Export not found	Runtime errors
NG0500–NG0505	Hydration mismatch	SSR common issues
NG0750	@defer requires standalone dependencies	Runtime errors
NG0912	Component ID generation collision	Build & tooling
NG0955	Duplicate keys in @for track	Runtime errors
NG0956	Tracking expression caused re-creation of the entire collection	Runtime errors
NG02200	Cannot find a differ	Runtime errors
NG04002	Cannot match any routes	Runtime errors
NG05104	Root element was not found	Build & tooling

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Symptom index (no error code)

What you observe	Likely area
The view does not update after data changes	Change detection
The app becomes slower the longer it runs	Memory leaks
The list re-renders completely on every update	Runtime errors — track
API calls fire twice	SSR common issues
A stale HTTP response overwrites a newer one	RxJS patterns
Build succeeds locally but fails in CI	Build & tooling
Bundle size exploded after a small change	Build & tooling

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General debugging toolkit

Enable detailed errors in development

Angular error codes link to angular.dev/errors/NGxxxx. Always read that page first — it is short and precise.

Angular DevTools

The browser extension gives you:

• Component tree with current inputs, outputs and signal values.
• Profiler showing which component triggered each change detection cycle and how long it took.
• Injector tree, invaluable for NG0201 and NG0200.

Runtime introspection from the console

// Select a DOM node in the Elements panel, then:
ng.getComponent($0);          // Component instance
ng.getContext($0);            // Template context (e.g. @for item)
ng.getOwningComponent($0);    // Parent component
ng.applyChanges(ng.getComponent($0)); // Force change detection

Narrow the surface

When the cause is unclear, bisect:

1. Comment out half of the template.
2. Replace real data with a hardcoded literal.
3. Remove OnPush temporarily — if the problem disappears, it is a change detection issue.
4. Remove provideClientHydration() temporarily — if it disappears, it is a hydration issue.

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Pages in this section

• Runtime errors
• Dependency Injection errors
• Change detection issues
• Build & tooling issues