Mapped & Conditional Types

Preview — 3 of 10 questions

What type does TypeScript infer for direction?

javascript

const direction = "north";

A"north" — the literal type

Bstring

Cstring | undefined

Dnever

What does TypeScript know about value inside each branch?

javascript

function process(value: string | number) {
  if (typeof value === 'string') {
    // value is: ?
  } else {
    // value is: ?
  }
}

ABoth branches: string | number

BBoth branches: unknown

Cif branch: string | number; else branch: never

Dif branch: string; else branch: number

What does TypeScript narrow error to in the catch block?

javascript

try {
  riskyOperation();
} catch (error) {
  if (error instanceof Error) {
    console.log(error.message); // ?
  }
}

Aerror is unknown, so .message always errors even with instanceof

Berror in catch is typed as Error by default

CInside the if, TypeScript narrows error to Error — .message is safely accessible

Dinstanceof only works with custom classes, not built-in Error

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