A value is missing even though the TypeScript model appeared to allow the code. How would you model and narrow nullable or optional data correctly?

Separates optional, undefined, null, and invalid external data, then narrows values at the correct boundary.

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TL;DR

With strict null checking, null and undefined are distinct types that must be handled before using the value. An optional property may be absent and is read as possibly undefined; value: T | null says the property exists but may explicitly contain null. I model the real states and narrow them with a check, discriminated union, or validation rather than adding !.

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Whether the candidate can diagnose a false assumption in the model instead of hiding it with non-null assertions.

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Core answer
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Overview
The first question is whether the value is legitimately absent, not yet available, explicitly empty, or invalid at the boundary.
With strict null checking, null and undefined are distinct types that must be handled before using the value. An optional property may be absent and is read as possibly undefined; value: T | null says the property exists but may explicitly contain null. I model the real states and narrow them with a check, discriminated union, or validation rather than adding !.
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Stronger answer

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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

Using a non-null assertion to silence every warning.
Treating optional, undefined, null, and invalid as identical automatically.
Assuming an API response matches its TypeScript annotation.
Using truthiness when zero or an empty string is valid.

A value is missing even though the TypeScript model appeared to allow the code. How would you model and narrow nullable or optional data correctly?

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