Interface declaration syntax in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an interface named Person.

Typescript

interface [1] {
  name: string;
  age: number;
}

Drag options to blanks, or click blank then click option'

APerson

Bperson

Cclass

Dtype

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'person' instead of 'Person'.

Using 'class' or 'type' instead of an interface name.

The keyword interface is followed by the interface name, which should be Person with a capital P by convention.

2fill in blank

medium

Complete the code to add an optional property 'email' to the interface.

Typescript

interface User {
  id: number;
  username: string;
  [1]?: string;
}

Drag options to blanks, or click blank then click option'

Aemail?

Bemail!

Cemail

Demail:

Attempts:

3 left

💡 Hint

Common Mistakes

Putting '?' inside the property name like 'email?'.

Using 'email!' which is not valid syntax here.

To declare an optional property in an interface, add a question mark ? after the property name. Here, the property name is email, and the question mark is placed after it in the code.

3fill in blank

hard

Fix the error in the interface declaration by completing the code.

Typescript

interface Product {
  id: number;
  name: string[1]
  price: number;
}

Drag options to blanks, or click blank then click option'

A,

B.

C:

D;

Attempts:

3 left

💡 Hint

Common Mistakes

Using commas instead of semicolons.

Forgetting to put any punctuation at the end of property lines.

In TypeScript interfaces, each property declaration must end with a semicolon ;. The missing semicolon after name: string causes a syntax error.

4fill in blank

hard

Fill both blanks to declare an interface with a method that returns a string.

Typescript

interface Logger {
  log[1]: [2];
}

Drag options to blanks, or click blank then click option'

A()

Bstring

Cnumber

D{}

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting parentheses after method name.

Using incorrect return types like number or empty braces.

The method log needs parentheses () to show it is a function, and the return type is string.

5fill in blank

hard

Fill all three blanks to declare an interface with an index signature for string keys and number values.

Typescript

interface Scores {
  [[1]: [2]]: [3];
}

Drag options to blanks, or click blank then click option'

Akey

Bstring

Cnumber

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid variable names or types inside the brackets.

Mixing up key and value types.

An index signature uses a variable name (like key) with a type (here string) inside square brackets, followed by the value type (number) after the colon.

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