Declaration merging for interfaces in Typescript - Mini Project: Build & Apply

Declaration merging for interfaces

📖 Scenario: Imagine you are building a simple user profile system. You want to define a user interface that can be extended later with more details without changing the original interface.

🎯 Goal: Learn how to use declaration merging to extend interfaces in TypeScript by creating multiple interface declarations with the same name and combining their properties.

📋 What You'll Learn

Create an interface called User with two properties: name (string) and age (number).

Add a second declaration of the User interface with an additional property: email (string).

Create a variable user1 of type User with all three properties filled.

Print the user1 object to the console.

💡 Why This Matters

🌍 Real World

In real projects, you often need to extend existing interfaces from libraries or your own code without modifying the original interface. Declaration merging lets you do this cleanly.

💼 Career

Understanding declaration merging is useful for working with large TypeScript codebases, especially when integrating third-party libraries or adding features incrementally.

1

Create the initial User interface

Create an interface called User with two properties: name of type string and age of type number.

Typescript

// Your code here

Hint

Use the interface keyword followed by the name User. Inside curly braces, add name: string; and age: number;.

2

Add a second declaration of the User interface

Add another interface declaration called User with one property: email of type string.

Typescript

interface User {
  // Your code here
}

Hint

Write a new interface User block below the first one and add email: string; inside it.

3

Create a variable of type User

Create a variable called user1 of type User and assign it an object with name set to "Alice", age set to 30, and email set to "[e-mail]".

Typescript

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

interface User {
  email: string;
}

// Your code here

Hint

Use const user1: User = { ... } and fill in the properties exactly as shown.

4

Print the user1 object

Write a console.log statement to print the user1 object.

Typescript

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

interface User {
  email: string;
}

const user1: User = {
  name: "Alice",
  age: 30,
  email: "[e-mail]"
};

// Your code here

Hint

Use console.log(user1); to display the object in the console.

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