When should Angular state use a signal, computed value, or effect?

Separates source state, derived state, and synchronization with external systems to prevent reactive loops.

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TL;DR

I use a writable signal for state the application owns and can change. I use computed for a value that can be derived from other signals, because Angular memoizes it and tracks its dependencies. I use an effect for a real side effect such as logging, storage, or coordinating an imperative API, not for copying one signal into another.

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Writable signals, computed signals, effects, state derivation, side effects, and ownership.

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Core answer first, then checkpoints toward a stronger answer.

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Core answer
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Overview
Writable signals store source state, computed signals derive values, and effects synchronize with non-reactive systems.
I use a writable signal for state the application owns and can change. I use computed for a value that can be derived from other signals, because Angular memoizes it and tracks its dependencies. I use an effect for a real side effect such as logging, storage, or coordinating an imperative API, not for copying one signal into another.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
For example, a selected product ID can be writable and the selected product can be computed from the product list. Mirroring that product into another signal with an effect introduces timing and ownership questions that the computed value avoids. Effects track the signals read during their latest execution, so conditional reads change dependencies. I keep each effect small, give it a clear external destination, and clean up resources it creates.

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Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Using an effect to maintain ordinary derived state.
Creating feedback loops through signal writes.
Reading incidental values in an effect and accidentally tracking them.

When should Angular state use a signal, computed value, or effect?

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