JuniorTheoryCommon

What does tsc report for this file?

TypeScript

type User = { name: string; age?: number };
const u: User = { name: "Ann" };
console.log(u.age + 1);

An error: u.age may be undefined

Expecting tsc to let u.age + 1 through and print NaN: an optional property reads as number | undefined, so arithmetic on it needs a check or a default.
Expecting the literal to need age: the ? makes the property optional, so { name: "Ann" } is a valid User; only using age needs care.

JuniorTheoryCommon

TypeScript

type User = { name: string; age?: number };
const u: User = { name: "Ann" };
console.log(u.age + 1);

An error: u.age may be undefined

Expecting tsc to let u.age + 1 through and print NaN: an optional property reads as number | undefined, so arithmetic on it needs a check or a default.
Expecting the literal to need age: the ? makes the property optional, so { name: "Ann" } is a valid User; only using age needs care.