Why would you prefer `unknown` over `any` for data entering a TypeScript application?

Use unknown safely at TypeScript boundaries

Explains why external data needs runtime validation even when the application uses TypeScript.

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TL;DR

any disables type checking for the value and lets unsafe operations spread through the code. unknown says that a value exists but its shape is not trusted yet, so I have to narrow or validate it before reading properties. For a small value I can use type guards. For an API payload I usually parse it with a runtime schema and convert it into the application's trusted type.

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TypeScript trust boundaries, unknown, any, type narrowing, runtime validation, API contracts, and safe error handling.

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Why this question matters
TypeScript protects code only after data has been given a trustworthy shape. API responses, browser storage, URL values, and third-party messages arrive at runtime and can violate compile-time assumptions.
any disables type checking for the value and lets unsafe operations spread through the code. unknown says that a value exists but its shape is not trusted yet, so I have to narrow or validate it before reading properties. For a small value I can use type guards. For an API payload I usually parse it with a runtime schema and convert it into the application's trusted type.
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A TypeScript annotation or as SomeType assertion does not validate JSON because types are erased at runtime. I keep the untrusted value at the transport boundary, validate required fields and useful constraints, then return either a typed result or a structured parse error. That prevents malformed data from reaching components as if it were valid. It also gives the UI a deliberate failure state instead of a later undefined-property crash.

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Details

Casting JSON with as and treating the cast as validation.
Using any in an API client and spreading it through component props.
Validating only that an object is non-null.
Showing raw parse details or sensitive payloads to users.

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