Property does not exist on type.
Three fixes, three causes.

// ─── The error ───────────────────────────────
interface User {
id: string;
email: string;
}

const user: User = { id: '1', email: 'a@x.com' };
// console.log(user.name);
// ❌ Property 'name' does not exist on type 'User'

// ─── Fix 1: add the property ─────────────────
interface User2 {
id: string;
email: string;
name: string; // ← added
}

const user2: User2 = { id: '1', email: 'a@x.com', name: 'Alice' };
console.log(user2.name); // ✅ no error

// ─── Fix 1b: optional property ───────────────
interface User3 {
id: string;
email: string;
name?: string; // ← optional — may be missing at runtime
}

const user3: User3 = { id: '1', email: 'a@x.com' };
// console.log(user3.name.length); // ❌ 'name' possibly undefined
console.log(user3.name?.length); // ✅ optional chaining

// ─── Fix 1c: when the property isn't known upfront
// If the response shape varies, model it as a union:
interface BaseUser {
id: string;
email: string;
}
interface PersonalUser extends BaseUser {
kind: 'personal';
name: string;
}
interface BusinessUser extends BaseUser {
kind: 'business';
companyName: string;
}
type AnyUser = PersonalUser | BusinessUser;

function displayName(u: AnyUser): string {
if (u.kind === 'personal') return u.name; // ✅ narrowed
return u.companyName; // ✅ narrowed
}

console.log(user2.name, user3.name, displayName({
id: '2', email: 'b@x.com', kind: 'personal', name: 'Bob'
}));

// ─── The error ───────────────────────────────
type Shape =
| { kind: 'circle'; radius: number }
| { kind: 'square'; side: number };

function area(s: Shape): number {
// return s.radius;
// ❌ Property 'radius' does not exist on type 'Shape'.
// Property 'radius' does not exist on type '{ kind: "square"; side: number }'.

// ─── Fix 1: discriminate by kind ──────────
if (s.kind === 'circle') return Math.PI * s.radius ** 2; // ✅ narrowed
return s.side ** 2; // ✅ narrowed
}

// ─── Fix 2: use in for shape-based narrowing
function describe(s: Shape): string {
if ('radius' in s) return circle with r=${s.radius}square with side=${s.side}; // ✅ narrowed
}

// ─── Fix 3: typeof narrowing for primitive unions
function process(x: string | number): string {
// return x.toUpperCase();
// ❌ Property 'toUpperCase' does not exist on type 'string | number'

if (typeof x === 'string') return x.toUpperCase(); // ✅ narrowed to string
return x.toFixed(2); // ✅ narrowed to number
}

// ─── Fix 4: custom type guard ────────────────
interface Circle { radius: number }
interface Square { side: number }

function isCircle(s: Circle | Square): s is Circle {
return 'radius' in s;
}

function area2(s: Circle | Square): number {
if (isCircle(s)) return Math.PI * s.radius ** 2; // ✅ narrowed
return s.side ** 2;
}

console.log(area({ kind: 'circle', radius: 2 }));
console.log(describe({ kind: 'square', side: 3 }));
console.log(process(3.14));

// ─── The error: unknown JSON shape ───────────
interface User {
id: string;
name: string;
}

const data: User = JSON.parse('{"id":"1","name":"Alice","age":28}');
// const age = data.age;
// ❌ Property 'age' does not exist on type 'User'.
// The JSON has it, but your type doesn't.

// ─── Fix 1: extend the type (if the field is known)
interface User2 {
id: string;
name: string;
age?: number; // optional — response may or may not include it
}

const data2: User2 = JSON.parse('{"id":"1","name":"Alice","age":28}');
console.log(data2.age); // ✅ optional access

// ─── Fix 2: treat as a dictionary ────────────
const dict: Record<string, unknown> = JSON.parse('{"id":"1","name":"Alice","age":28}');
console.log(dict.age); // ✅ allowed — any key returns unknown

// ─── Better fix: validate at the boundary ────
// Don't cast JSON — narrow it with a type guard.
function isUser(x: unknown): x is User2 {
if (typeof x !== 'object' || x === null) return false;
const u = x as Record<string, unknown>;
return typeof u.id === 'string' && typeof u.name === 'string';
}

const raw: unknown = JSON.parse('{"id":"1","name":"Alice","age":28}');
if (isUser(raw)) {
// raw is User2 here — age is number | undefined
console.log(raw.name, raw.age);
}

console.log(data2.age, dict.age);