Declaration merging for interfaces in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of merged interface properties

What is the output of the following TypeScript code when compiled and run with Node.js (using ts-node or similar)?

Typescript

interface User {
  name: string;
}
interface User {
  age: number;
}
const user: User = { name: "Alice", age: 30 };
console.log(user);

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

B TypeError: Cannot read property 'name' of undefined

C SyntaxError: Duplicate identifier 'User'

D { name: 'Alice' }

💡 Hint

Think about how TypeScript merges interface declarations with the same name.

In TypeScript, interfaces with the same name are merged. So the final User interface has both 'name' and 'age' properties. The object 'user' correctly implements both, so the console.log outputs the full object.

❓ Predict Output

intermediate

Merged interface method call output

What will be printed when running this TypeScript code?

Typescript

interface Logger {
  log(message: string): void;
}
interface Logger {
  error(message: string): void;
}
const logger: Logger = {
  log: (msg) => console.log('LOG:', msg),
  error: (msg) => console.error('ERROR:', msg)
};
logger.log('Hello');
logger.error('Oops');

A SyntaxError: Duplicate interface 'Logger'

B TypeError: logger.error is not a function

C LOG: Hello\nERROR: Oops

D LOG: Hello

💡 Hint

Interfaces with the same name combine their members.

The Logger interface merges both methods. The object implements both, so calling logger.log and logger.error prints both messages.

🔧 Debug

advanced

Why does this merged interface cause a type error?

Given the following code, why does TypeScript report an error on the variable 'person'?

Typescript

interface Person {
  name: string;
}
interface Person {
  name: number;
}
const person: Person = { name: 'Bob' };

A Because the variable 'person' is missing the 'age' property.

B Because the 'name' property has conflicting types (string and number) in merged interfaces.

C Because interfaces cannot be declared twice with the same name.

D Because the object literal is missing a type annotation.

💡 Hint

Check the types of the 'name' property in both interface declarations.

When interfaces merge, properties with the same name must have compatible types. Here, 'name' is string in one and number in the other, causing a conflict and a type error.

📝 Syntax

advanced

Which interface merging syntax is invalid?

Which of the following interface declarations will cause a TypeScript syntax error due to invalid merging?

A

interface Settings { theme: string; }
interface Settings { theme: number; }

B

interface Config { url: string; }
interface Config { timeout?: number; }

C

interface Data { id: number; }
interface Data { value: string; }

D

interface Options { debug: boolean; }
interface Options { debug: boolean; }

💡 Hint

Look for conflicting property types in the same interface name.

Option A declares 'theme' as string and number in the same interface name, causing a type conflict error. The others merge without syntax errors.

🚀 Application

expert

How many properties does the merged interface have?

Consider these interface declarations in TypeScript. How many properties does the merged interface 'Product' have?

Typescript

interface Product {
  id: number;
  name: string;
}
interface Product {
  description?: string;
  price: number;
}
interface Product {
  price: number;
  stock: number;
}

A 4

B 3

C 6

D 5

💡 Hint

Count unique property names after merging all interfaces.

The merged interface has properties: id, name, description, price, and stock. 'price' is repeated but counts once. So total 5 properties.

Practice

1. What happens when two interfaces with the same name are declared in TypeScript?

easy

A. Their properties merge into a single interface.

B. The second interface overwrites the first one.

C. A syntax error occurs due to duplicate names.

D. Only the first interface is used, others ignored.

Solution

1. Step 1: Understand interface declaration merging

   In TypeScript, interfaces with the same name combine their properties into one interface.

2. Step 2: Effect of merging

   This allows extending interfaces without overwriting or errors.

3. Final Answer:

   Their properties merge into a single interface. -> Option A

4. Quick Check:

   Declaration merging = properties combined [OK]

Hint: Same name interfaces combine properties automatically [OK]

Common Mistakes:

• Thinking second interface overwrites the first
• Expecting a syntax error on duplicate names
• Assuming only the first interface is used

2. Which of the following is the correct syntax to merge two interfaces named Person in TypeScript?

easy

A. class Person { name: string } class Person { age: number }

B. interface Person { name: string } interface Person { age: number }

C. type Person = { name: string } type Person = { age: number }

D. interface Person { name: string } type Person = { age: number }

Solution

1. Step 1: Identify valid interface merging syntax

   Only multiple interface declarations with the same name merge properties.

2. Step 2: Check each option

   interface Person { name: string } interface Person { age: number } uses two interface Person declarations, which is correct. Options B and C mix types and interfaces or duplicate types, which do not merge. class Person { name: string } class Person { age: number } uses classes, which do not merge.

3. Final Answer:

   interface Person { name: string } interface Person { age: number } -> Option B

4. Quick Check:

   Multiple interface declarations = merge [OK]

Hint: Only interfaces with same name merge, not types or classes [OK]

Common Mistakes:

• Trying to merge types instead of interfaces
• Using classes instead of interfaces
• Mixing type and interface declarations

3. What will be the output of the following TypeScript code?

interface Car { brand: string; }
interface Car { year: number; }
const myCar: Car = { brand: "Toyota", year: 2020 };
console.log(myCar.year);

medium

A. undefined

B. Runtime error

C. Compilation error

D. 2020

Solution

1. Step 1: Understand interface merging effect

   The two Car interfaces merge their properties, so Car has both brand and year.

2. Step 2: Analyze object and output

   The object myCar correctly has both properties. Logging myCar.year outputs 2020.

3. Final Answer:

   2020 -> Option D

4. Quick Check:

   Interface merge allows all properties [OK]

Hint: Merged interfaces combine properties, so all exist in object [OK]

Common Mistakes:

• Expecting a compilation error due to duplicate interface
• Thinking only first interface properties exist
• Confusing runtime and compile-time errors

4. Identify the error in the following TypeScript code that attempts interface merging:

interface User { id: number; }
interface User { name: string; }
const user: User = { id: 1 };

medium

A. Cannot assign number to 'id' property.

B. Duplicate interface declarations cause syntax error.

C. Missing property 'name' in object 'user'.

D. Interface merging does not work with 'const' declarations.

Solution

1. Step 1: Check interface merging and properties

   The two User interfaces merge, so User requires both id and name.

2. Step 2: Analyze object assignment

   The object user only has id, missing name, causing a type error.

3. Final Answer:

   Missing property 'name' in object 'user'. -> Option C

4. Quick Check:

   All merged properties required in object [OK]

Hint: Merged interfaces require all properties in objects [OK]

Common Mistakes:

• Thinking duplicate interfaces cause syntax errors
• Ignoring missing properties in merged interface
• Assuming 'const' affects interface merging

5. Given two interface declarations:

interface Product { id: number; }
interface Product { description?: string; price: number; }

Which of the following objects correctly implements the merged Product interface?

hard

A. { id: 101, price: 29.99 }

B. { id: 101, description: "Book" }

C. { price: 29.99, description: "Book" }

D. { id: 101 }

Solution

1. Step 1: Combine interface properties

   The merged Product interface requires id (number) and price (number), with optional description.

2. Step 2: Check each object against required properties

   { id: 101, price: 29.99 } has id and price, valid. { id: 101, description: "Book" } misses price. { price: 29.99, description: "Book" } misses id. { id: 101 } misses price.

3. Final Answer:

   { id: 101, price: 29.99 } -> Option A

4. Quick Check:

   Required properties must be present [OK]

Hint: All required merged properties must be in object [OK]

Common Mistakes:

• Ignoring required properties when optional ones exist
• Assuming optional properties are required
• Missing required properties in object