When would you use input, output, or model signals in Angular?

Distinguishes one-way data, events, and deliberate two-way component contracts without hiding ownership.

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TL;DR

I use an input when the parent owns a value and the child reads it, and an output when the child reports an event that the parent decides how to handle. A model signal is for a deliberately writable two-way contract, such as a reusable control whose value belongs to the caller but can be changed through the child. I do not use model merely to avoid writing an output because that makes ownership less clear.

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Practice focus

Signal inputs, outputs, model signals, component contracts, ownership, and two-way binding.

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Answer path

Core answer first, then checkpoints toward a stronger answer.

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Core answer
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Overview
The choice communicates who owns the value and how changes cross the component boundary.
I use an input when the parent owns a value and the child reads it, and an output when the child reports an event that the parent decides how to handle. A model signal is for a deliberately writable two-way contract, such as a reusable control whose value belongs to the caller but can be changed through the child. I do not use model merely to avoid writing an output because that makes ownership less clear.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
Inputs can be required, transformed, and combined with computed state, which often removes lifecycle bookkeeping. Outputs describe events such as saved or selectionChanged; I name them after what happened rather than exposing internal button clicks. A model is appropriate when both sides genuinely participate in one value, but commands, server mutations, and unrelated state changes still deserve explicit events. This keeps the template convenient without turning every child into a hidden state writer.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

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

Details

Copying an input into local writable state without a clear divergence rule.
Using a model signal for commands or unrelated side effects.
Naming outputs after DOM events instead of domain meaning.

When would you use input, output, or model signals in Angular?

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