What is the difference between any and unknown in TypeScript, and when do you reach for each?

any switches off type checking for that value, so every property access and call compiles even when it is wrong. unknown accepts any value coming in but forces you to narrow it with a typeof check, a type guard or a schema parse before you can use it. Reach for unknown at trust boundaries like fetch responses and JSON.parse. Reach for any almost never.

My rule is that any is a bug I have not found yet. If I write any, I have told the compiler to stop helping and the failure surfaces at runtime in a user's browser instead. unknown is the honest version: it says I do not know what this is, and the compiler makes me prove the shape before I touch it. In practice that means anything crossing a trust boundary starts as unknown. A fetch response, a webhook body, whatever comes out of JSON.parse or localStorage. At a fintech I worked at we hand wrote our API response types and they drifted from the backend within about two months, so a field that became nullable started crashing a table render in production. We moved to parsing with zod at the boundary and inferring the TypeScript types from the schema, and that whole class of bug disappeared. The only place I still allow any is inside a tightly scoped generic helper, and even there unknown plus a guard usually does the job.