TypeScript Fundamentals — Series 3

Preview — 3 of 10 questions

What does the emitted JavaScript contain, and what is logged?

javascript

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

const user: User = { id: 1, name: "Ada" };
console.log(typeof user);

AThe emitted JS keeps the interface declaration, and "User" is logged

BThe emitted JS keeps the type annotation : User on the constant, and "User" is logged

CThe interface and the : User annotation are both erased, and "object" is logged

DCompile-time error — typeof cannot be used on a value with an interface type

javascript

let score: number = 10;
score = "10";

ACompiles fine — TypeScript converts "10" to the number 10

BCompile-time error — score is annotated number, so a string isn't assignable to it

CCompiles fine, but score becomes NaN at runtime

DCompiles fine — the annotation only applies to the initial value

javascript

const ids: Array<number> = [1, 2, 3];
ids.push(4);
ids.push("5");

ABoth push calls compile — arrays accept any value at runtime

BCompile-time error on ids.push(4) — Array<number> is read-only

CBoth push calls compile, but "5" is coerced to the number 5

Dids.push(4) compiles; ids.push("5") is a compile-time error — the element type is number

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