Interface declaration syntax in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an interface in TypeScript?

An interface in TypeScript defines a structure for objects, specifying what properties and methods they should have without implementing them.

beginner

How do you declare a simple interface with two properties: name (string) and age (number)?

Use the interface keyword followed by the interface name and property types:

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

intermediate

Can interfaces in TypeScript include methods? How do you declare one?

Yes, interfaces can include method signatures without implementation. For example:

interface Person {
  greet(): void;
}

This means any object implementing Person must have a greet method that returns nothing.

intermediate

What does it mean when a property in an interface is marked optional with a question mark (?)?

An optional property means objects implementing the interface may or may not have that property. For example:

interface Person {
  middleName?: string;
}

The middleName property can be missing.

intermediate

How do you extend an interface from another interface in TypeScript?

Use the extends keyword to create a new interface that includes all properties of the original plus new ones:

interface Employee extends Person {
  employeeId: number;
}

Which keyword is used to declare an interface in TypeScript?

A class

B interface

C type

D struct

The interface keyword is used to declare interfaces in TypeScript.

How do you mark a property as optional in an interface?

A Use the keyword optional before the property

B Set the property type to undefined

C Add a question mark (?) after the property name

D Use the keyword nullable

A question mark (?) after the property name marks it as optional.

What does this interface declaration mean?

interface Car {
  drive(): void;
}

A drive is a method that must return a string

B drive is a property of type void

C drive is a variable to store a function

D Any object with a drive method that returns nothing matches this interface

The interface requires a drive method that returns nothing (void).

How do you create an interface that includes all properties of another interface plus new ones?

A Use the extends keyword

B Use the implements keyword

C Use the include keyword

D Use the inherit keyword

The extends keyword is used to inherit properties from another interface.

Which of these is a valid interface declaration?

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

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

C interface User (id: number, name: string)

D interface User { id => number; name => string; }

Option A uses correct TypeScript interface syntax.

Explain how to declare an interface with optional properties and methods in TypeScript.

Think about how to mark properties optional and how methods are declared without code.

Describe how interface inheritance works in TypeScript and why it is useful.

Consider how one interface can build on another.

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