TypeScript any, unknown, and never types when to use each

any — escape hatch, use sparingly

any turns off all type checking for a variable. Avoid it — it defeats the purpose of TypeScript:

let val: any = "hello";
val = 42;          // fine
val.toUpperCase(); // no error — but crashes if val is 42

unknown — safe alternative to any

unknown accepts any value but forces you to check the type before using it:

function process(input: unknown) {
  if (typeof input === "string") {
    console.log(input.toUpperCase()); // safe
  }
  // input.toUpperCase() here would be a compile error
}

Use unknown for values from external sources: API responses, parsed JSON, user input.

never — code that should not be reached

never represents a value that can never exist. TypeScript assigns it to unreachable code paths:

function fail(msg: string): never {
  throw new Error(msg);
}

function assertExhaustive(x: never): never {
  throw new Error("Unhandled case: " + x);
}

The exhaustive check pattern is powerful with discriminated unions — if you add a new variant and forget to handle it, TypeScript flags the never assignment as an error before runtime.