Avoiding any, Understanding never, and Assertions

any opts out of checking and can spread unsafety through a codebase. unknown is a safer top type. never represents an impossible value, often appearing when a function always throws or when a union has been exhaustively narrowed. Type assertions such as value as User tell the compiler to trust you; they do not convert or validate the value.

Use assertions only when you have evidence the compiler cannot express, such as knowledge from a validated boundary or a DOM lookup with a known element type. Keep assertions local, and avoid double assertions such as as unknown as X except in carefully justified interop code.

Use never for exhaustive checks

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; side: number };

function area(shape: Shape): number {
  switch (shape.kind) {
    case "circle": return Math.PI * shape.radius ** 2;
    case "square": return shape.side ** 2;
    default: {
      const impossible: never = shape;
      return impossible;
    }
  }
}

💡 After both variants are handled, shape should be never. If another shape is added and not handled, the assignment reports an error.

Key Takeaways

any disables checks; unknown requires narrowing.
never is useful for impossible states and exhaustiveness.
Assertions change the compiler's view, not the runtime value.

Knowledge Check

Does `value as User` validate the value at runtime?

Avoiding any, Understanding never, and Assertions

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