Type alias vs inline types in Typescript - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function using type alias vs inline type

What is the output of this TypeScript code when calling printUserInfo(user)?

Typescript

type User = { name: string; age: number };

const user: User = { name: "Alice", age: 30 };

function printUserInfo(u: User) {
  console.log(`Name: ${u.name}, Age: ${u.age}`);
}

printUserInfo(user);

A Name: undefined, Age: undefined

B Name: Alice, Age: undefined

C Name: Alice, Age: 30

D Name: undefined, Age: 30

💡 Hint

Check how the type alias User is used to type the variable and function parameter.

The type alias User defines the shape of the object. The variable user matches this shape, so the function prints the correct values.

❓ Predict Output

intermediate

Output difference between inline type and type alias

What will this code output when calling printUserInfo(user)?

Typescript

const user = { name: "Bob", age: 25 };

function printUserInfo(u: { name: string; age: number }) {
  console.log(`Name: ${u.name}, Age: ${u.age}`);
}

printUserInfo(user);

A Name: Bob, Age: 25

B Name: undefined, Age: undefined

C Name: Bob, Age: undefined

D Name: undefined, Age: 25

💡 Hint

Inline types work the same as type aliases for typing function parameters.

The inline type defines the expected shape directly in the function parameter. The object matches this shape, so the output is correct.

🔧 Debug

advanced

Why does this code cause a TypeScript error?

This code causes a TypeScript error. What is the reason?

Typescript

type Person = { name: string; age: number };

const person: Person = { name: "Eve" };

function greet(p: Person) {
  console.log(`Hello, ${p.name}`);
}

greet(person);

A The variable 'person' is not declared with 'const'.

B The function parameter 'p' is missing the 'age' property.

C Type alias 'Person' cannot be used for object typing.

D Missing property 'age' in the object assigned to 'person'.

💡 Hint

Check if the object matches all properties defined in the type alias.

The object assigned to 'person' lacks the required 'age' property, causing a TypeScript error.

🧠 Conceptual

advanced

Difference between type alias and inline type in function parameters

Which statement best describes the difference between using a type alias and an inline type for function parameters in TypeScript?

A Type aliases cannot be used for function parameters, only inline types can.

B Type aliases improve code readability and reusability, while inline types are defined directly and used once.

C Inline types automatically infer types from variables, type aliases do not.

D Inline types are faster at runtime than type aliases.

💡 Hint

Think about how type aliases help organize code.

Type aliases let you name a type and reuse it, making code cleaner. Inline types are written directly where needed and are not reusable.

📝 Syntax

expert

Which option causes a syntax error?

Which of the following TypeScript snippets causes a syntax error?

A

type User = { name: string; age: number }
const user: User = { name: "Sam", age: 40 };

B

function greet(user: { name: string; age: number }) {
  console.log(user.name);
}

C

type User = { name: string; age: number };
const user: User = { name: "Sam", age: 40 };

D const user: { name: string; age: number } = { name: "Sam", age: 40 };

💡 Hint

Check for missing semicolons or braces in type alias declarations.

Option A is missing a semicolon in the type alias declaration, causing a syntax error.