TypeScript MCQs - Narrowing, Declarations and Tooling (Set 6)

Q1. What is the type of v after the call?

function assertIsString(v: unknown): asserts v is string {
  if (typeof v !== "string") throw new Error("Not a string");
}
assertIsString(v);

a) string

b) never

c) any

d) unknown

Q2. What does a signature like function assert(cond: unknown, msg?: string): asserts cond mean?

a) If the call returns, the condition passed in is treated as true for narrowing afterwards

b) The function returns a boolean

c) cond must be a boolean literal

d) It only works in tests

Q3. This code gives error TS2775. Why?

const assertStr = (v: unknown): asserts v is string => { ... };
assertStr(input);

a) Arrow functions cannot throw

b) Assertion functions must be called through a name with an explicit type annotation, which an inferred const does not have

c) asserts only works with classes

d) input must be any

Q4. From TypeScript 5.5, what is the type of xs?

const xs = [1, undefined, 2].filter(x => x !== undefined);

a) undefined[]

b) number[]

c) (number | undefined)[]

d) any[]

Q5. Why should an assertNever(x: never) helper also throw at runtime?

a) It is required by the compiler

b) Data from APIs or JavaScript callers can still contain values the types did not expect, so the default branch can run

c) never values are always undefined

d) Throwing improves performance

Q6. What happens with these two declarations?

interface Config { port: string }
interface Config { port: number }

a) port becomes string | number

b) port becomes never

c) Compile error: subsequent property declarations must have the same type

d) The second one wins

Q7. What does this code do?

function greet(name: string) { return "Hi " + name; }
namespace greet {
  export const version = "1.0";
}

a) Overwrites the function

b) Creates two separate values

c) Compile error: duplicate identifier

d) Merges the namespace into the function, so greet.version is typed and available

Q8. To add a user property to Express's Request type, what must the augmentation file contain?

a) A copy of all Express types

b) A declare module block for the Express types package that re-opens the Request interface, inside a file that is itself a module

c) A runtime assignment to Request.prototype

d) A class that extends Request

Q9. Why is export {} often added to a file containing declare global { interface Window { analytics: Analytics } }?

a) It removes the global

b) It exports the Window type

c) It is needed for tree shaking

d) declare global is only allowed in a module, and export {} turns the file into a module

Q10. What does declare const __APP_VERSION__: string; do?

a) Defines a constant enum

b) Tells the compiler that a global with this type exists, without emitting any JavaScript

c) Imports the value from package.json

d) Creates the variable at runtime

Q11. A project gets "Cannot find module './logo.svg'". What declaration fixes the import?

a) import type svg from "svg"

b) declare module "*.svg" { const src: string; export default src; } in a .d.ts file

c) Set allowJs to true

d) Rename the file to logo.ts

Q12. Which statement about a .d.ts file is correct?

a) It must be imported in every file

b) It can contain function bodies

c) It contains only type declarations and produces no JavaScript output

d) It is compiled into a .js file

Q13. In TypeScript project references, what is required in a referenced project and how is the build run?

a) composite: true in the referenced project, and tsc --build (tsc -b) to build projects in dependency order

b) Nothing special; run tsc

c) strict: false and tsc --watch

d) A webpack config

Q14. Under strict mode, what is the type of e in try { ... } catch (e) { ... }?

a) any

b) Error

c) never

d) unknown

Q15. Why is x.length still an error here?

function f(x: string[] | null) {
  if (typeof x === "object") {
    x.length;
  }
}

a) Arrays are not objects

b) typeof cannot narrow unions

c) typeof null is "object", so x is still string[] | null inside the block

d) length does not exist on arrays