What types exist only at compile time in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Types that exist only at compile time in TypeScript

Which of the following types exist only during TypeScript's compile time and do not appear in the generated JavaScript code?

A Interfaces and type aliases

B Classes and enums

C Functions and variables

D Objects and arrays

💡 Hint

Think about what TypeScript removes when it compiles to JavaScript.

Interfaces and type aliases are used only for type checking during compile time. They do not produce any JavaScript code.

Classes, enums, functions, variables, objects, and arrays exist at runtime in JavaScript.

❓ Predict Output

intermediate

Output of TypeScript code with interface

What will be the output when this TypeScript code is compiled and run as JavaScript?

Typescript

interface Person {
  name: string;
  age: number;
}

const user: Person = { name: 'Alice', age: 30 };
console.log(user);

A { name: 'Alice', age: 30 }

B undefined

C Error: Interface Person does not exist at runtime

D ReferenceError: user is not defined

💡 Hint

Interfaces disappear after compilation, but objects remain.

The interface is removed during compilation. The object user remains and is logged as expected.

❓ Predict Output

advanced

Type alias usage and runtime presence

Given this TypeScript code, what will be the output when compiled and run as JavaScript?

Typescript

type ID = number | string;

const userId: ID = 123;
console.log(typeof userId);

A TypeError at runtime

B "ID"

C "string"

D "number"

💡 Hint

Type aliases do not exist at runtime, only the value does.

The type alias ID is removed during compilation. The variable userId holds the number 123, so typeof userId is "number".

🧠 Conceptual

advanced

Which TypeScript constructs are erased at compile time?

Select the TypeScript constructs that are completely erased and do not exist in the emitted JavaScript code.

A Enums and namespaces

B Interfaces and type aliases

C Classes and functions

D Variables and constants

💡 Hint

Think about what is used only for type checking.

Interfaces and type aliases are only for type checking and are removed during compilation.

Enums, namespaces, classes, functions, variables, and constants generate JavaScript code.

🧠 Conceptual

expert

Understanding TypeScript's type system and runtime

Which statement correctly describes the relationship between TypeScript types and JavaScript runtime values?

A Interfaces are converted to JavaScript classes at runtime.

B All TypeScript types exist at runtime as JavaScript objects.

C TypeScript types are only for compile-time checks and do not exist at runtime.

D Type aliases generate JavaScript functions during compilation.

💡 Hint

Consider what happens to types after TypeScript compiles to JavaScript.

TypeScript types are erased after compilation and only help during development for type safety.

Interfaces and type aliases do not produce JavaScript code.