Interface declaration syntax in Typescript - Mini Project: Build & Apply

Interface declaration syntax

📖 Scenario: You are creating a simple program to store information about a book in a library system.

🎯 Goal: Build a TypeScript interface to define the structure of a book object and use it to create a book variable.

📋 What You'll Learn

Create an interface named Book with properties title (string), author (string), and year (number).

Create a variable named myBook of type Book with the exact values: title: 'The Great Gatsby', author: 'F. Scott Fitzgerald', year: 1925.

Print the myBook variable to the console.

💡 Why This Matters

🌍 Real World

Interfaces help programmers define clear rules for data structures, making code easier to understand and less error-prone.

💼 Career

Knowing how to declare and use interfaces is essential for TypeScript developers working on large projects or collaborating in teams.

1

Create the Book interface

Write an interface named Book with three properties: title of type string, author of type string, and year of type number.

Typescript

// Your code here

Hint

Use the interface keyword followed by the interface name and curly braces to define properties with their types.

2

Create a myBook variable of type Book

Create a variable named myBook of type Book and assign it an object with title set to 'The Great Gatsby', author set to 'F. Scott Fitzgerald', and year set to 1925.

Typescript

interface Book {
  title: string;
  author: string;
  year: number;
}

// Your code here

Hint

Use const myBook: Book = { ... } with the exact property names and values.

3

Add a function to display book info

Write a function named displayBook that takes a parameter book of type Book and returns a string formatted as "Title: [title], Author: [author], Year: [year]" using template literals.

Typescript

interface Book {
  title: string;
  author: string;
  year: number;
}

const myBook: Book = {
  title: 'The Great Gatsby',
  author: 'F. Scott Fitzgerald',
  year: 1925
};

// Your code here

Hint

Use a function with a typed parameter and return a formatted string using backticks and ${} placeholders.

4

Print the book information

Use console.log to print the result of calling displayBook with myBook as the argument.

Typescript

interface Book {
  title: string;
  author: string;
  year: number;
}

const myBook: Book = {
  title: 'The Great Gatsby',
  author: 'F. Scott Fitzgerald',
  year: 1925
};

function displayBook(book: Book): string {
  return `Title: ${book.title}, Author: ${book.author}, Year: ${book.year}`;
}

// Your code here

Hint

Call console.log(displayBook(myBook)) to show the formatted book info.

Practice

1. What is the purpose of an interface in TypeScript?

easy

A. To import external modules

B. To execute code conditionally

C. To define the shape of an object with properties and methods

D. To create a new variable

Solution

1. Step 1: Understand the role of interfaces

   Interfaces describe how an object should look by specifying its properties and methods.

2. Step 2: Compare with other options

   Options A, B, and C describe unrelated concepts like control flow, variables, and modules.

3. Final Answer:

   To define the shape of an object with properties and methods -> Option C

4. Quick Check:

   Interface = Object shape definition [OK]

Hint: Interfaces define object structure, not code execution [OK]

Common Mistakes:

• Confusing interfaces with classes
• Thinking interfaces run code
• Mixing interfaces with variables

2. Which of the following is the correct syntax to declare an interface named Person with a string property name?

easy

A. interface Person { name = string; }

B. interface Person = { name: string; }

C. interface Person (name: string);

D. interface Person { name: string; }

Solution

1. Step 1: Recall interface declaration syntax

   Interfaces use the keyword interface, followed by the name and curly braces with property declarations using colon syntax.

2. Step 2: Check each option

   interface Person { name: string; } uses correct syntax: interface Person { name: string; }. Options B, C, and D have syntax errors like using equals, parentheses, or assignment instead of colon.

3. Final Answer:

   interface Person { name: string; } -> Option D

4. Quick Check:

   Interface syntax uses colon for properties [OK]

Hint: Use colon, not equals, to declare properties in interfaces [OK]

Common Mistakes:

• Using '=' instead of ':' for properties
• Using parentheses instead of braces
• Assigning values inside interface

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

interface Car {
  brand: string;
  year: number;
}

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

medium

A. Toyota 2020

B. undefined 2020

C. Error: Property 'brand' missing

D. Toyota undefined

Solution

1. Step 1: Understand the interface and object assignment

   The interface Car requires brand (string) and year (number). The object myCar correctly provides both.

2. Step 2: Analyze the console.log output

   Concatenating myCar.brand and myCar.year with a space results in Toyota 2020.

3. Final Answer:

   Toyota 2020 -> Option A

4. Quick Check:

   Correct object matches interface = "Toyota 2020" [OK]

Hint: Check if object matches interface properties exactly [OK]

Common Mistakes:

• Missing required properties in object
• Confusing property names
• Expecting runtime errors for correct code

4. Identify the error in this interface declaration and object assignment:

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

const user: User = { id: 1 };

medium

A. Interface name should be lowercase

B. Missing property 'name' in object assigned to User

C. Property 'id' should be a string, not number

D. Object should use parentheses instead of braces

Solution

1. Step 1: Check interface properties

   The interface User requires both id (number) and name (string).

2. Step 2: Check object assignment

   The object assigned to user only has id, missing the required name property, causing a TypeScript error.

3. Final Answer:

   Missing property 'name' in object assigned to User -> Option B

4. Quick Check:

   All interface properties must be present in object [OK]

Hint: All interface properties must appear in assigned objects [OK]

Common Mistakes:

• Ignoring missing required properties
• Thinking interface names must be lowercase
• Confusing property types

5. Given the interface declaration below, which object correctly implements Product with an optional description property?

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

hard

A. { id: 101, name: "Book" }

B. { id: 101, description: "A book" }

C. { name: "Book", description: "A book" }

D. { id: "101", name: "Book" }

Solution

1. Step 1: Understand optional properties in interfaces

   The description property is optional (marked with ?), so it can be omitted.

2. Step 2: Check each object against the interface

   { id: 101, name: "Book" } has required id (number) and name (string), no description, which is valid.
   { id: 101, description: "A book" } misses required name.
   { name: "Book", description: "A book" } misses required id.
   { id: "101", name: "Book" } has id as string instead of number.

3. Final Answer:

   { id: 101, name: "Book" } -> Option A

4. Quick Check:

   Optional properties can be omitted safely [OK]

Hint: Optional properties can be left out without error [OK]

Common Mistakes:

• Omitting required properties
• Using wrong types for properties
• Confusing optional with required