Testing & Code Quality

Runtime Validation

Runtime validation checks actual values while the program runs. It complements TypeScript by proving that untrusted input matches the type the rest of the code wants to use.

Validation is runtime evidence
Parsing is better than checking-and-continuing
Validation should report useful errors
Schemas reduce type drift
Validate as close to the boundary as possible

Small parser function

type User = { id: string; name: string }

function parseUser(value: unknown): User {
  if (typeof value !== "object" || value === null) {
    throw new Error("User must be an object")
  }

  const candidate = value as { id?: unknown; name?: unknown }
  if (typeof candidate.id !== "string") throw new Error("User.id must be string")
  if (typeof candidate.name !== "string") throw new Error("User.name must be string")

  return { id: candidate.id, name: candidate.name }
}

Manual parsing is fine for small shapes; schema libraries help as shapes grow.

Validation results
Style	Shape	Use when
Throwing parser	returns T or throws	Invalid input is exceptional at this layer
Result parser	success/error union	Caller should recover or report multiple errors
Boolean guard	value is T	Simple branching
Assertion function	asserts value is T	Abort current path on invalid input