Fix TypeScript 'Property does not exist on type' Error

Error TS2339 is TypeScript doing its job: you are reading a property it cannot prove exists. The right fix is almost never any — it is giving the compiler the type information it is missing.

Fix 1: Type Your API Responses

response.json() returns any/unknown. Tell TS what shape it is:

interface User { id: string; name: string; email: string; }

const res = await fetch("/api/user");
const user = (await res.json()) as User;
console.log(user.name); // ✅ known property

Fix 2: Narrow Union Types Before Access

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

function area(s: Shape) {
  if (s.kind === "circle") return Math.PI * s.radius ** 2; // narrowed
  return s.side ** 2;
}

Fix 3: Index Signatures for Dynamic Keys

const config: Record<string, string> = {};
config["theme"] = "dark"; // ✅ any string key allowed

Fix 4: Augment a Third-Party Type

When a library is missing a property you need (e.g. a custom field on the Express Request), declare it once via module augmentation rather than casting everywhere.