Type alias for objects in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing properties in a type alias object

What is the output of this TypeScript code when compiled and run with Node.js?

Typescript

type Person = { name: string; age: number };
const user: Person = { name: "Alice", age: 30 };
console.log(user.name + " is " + user.age + " years old.");

A Compilation error

B undefined is undefined years old.

C TypeError at runtime

D Alice is 30 years old.

💡 Hint

Look at how the object is created and how properties are accessed.

The type alias Person defines an object with name and age. The object user matches this type and has valid values. Accessing user.name and user.age prints the expected string.

🧠 Conceptual

intermediate

Understanding type alias with optional properties

Given this type alias, which statement is true about the object assignment below?

Typescript

type Car = { make: string; model: string; year?: number };
const myCar: Car = { make: "Toyota", model: "Corolla" };

A The object is valid because year is optional.

B The object is invalid because year is missing.

C The object is invalid because model is missing.

D The object is invalid because make is optional but missing.

💡 Hint

Check the question mark in the type alias definition.

The question mark after year means it is optional. So the object can omit year and still be valid.

🔧 Debug

advanced

Identify the error in this type alias usage

What error will this TypeScript code produce?

Typescript

type Point = { x: number; y: number };
const p: Point = { x: 10, y: "20" };

A Property 'y' is missing in object.

B Type 'string' is not assignable to type 'number'.

C No error, code runs fine.

D Type 'number' is not assignable to type 'string'.

💡 Hint

Look at the type of y in the object and in the type alias.

The type alias expects y to be a number, but the object assigns a string "20". This causes a type error.

📝 Syntax

advanced

Correct syntax for type alias with nested objects

Which option correctly defines a type alias for an object with a nested address object?

A type User = { name: string; address: { street: string; city: string } };

B type User = { name: string; address: [street: string, city: string] };

C type User = { name: string; address: (street: string, city: string) };

D type User = { name: string; address: string, city: string };

💡 Hint

Nested objects use curly braces, not arrays or parentheses.

Option A correctly uses an object type for address with street and city as strings.

🚀 Application

expert

Determine the number of keys in a type alias object at runtime

Given this type alias and object, what is the output of the code below?

Typescript

type Config = { host: string; port: number; secure?: boolean };
const serverConfig: Config = { host: "localhost", port: 8080 };
console.log(Object.keys(serverConfig).length);

A 3

B 1

C 2

D 0

💡 Hint

Optional properties not set do not appear in the object keys.

Only host and port are present in the object, so Object.keys returns 2 keys.