Why object types are needed in Typescript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an object type for a person with a name and age.

Typescript

type Person = { name: string; age: [1] };

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

Aboolean

Bnumber

Cstring

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' for age instead of 'number'.

Using 'boolean' which is true/false, not suitable here.

The age property should be a number because age is a numeric value.

2fill in blank

medium

Complete the code to create a variable with the Person type.

Typescript

const user: [1] = { name: 'Alice', age: 30 };

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

APerson

Bobject

Cstring

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' or 'object' instead of the custom type 'Person'.

Using 'any' which disables type checking.

The variable user should be typed as Person to match the object structure.

3fill in blank

hard

Fix the error in the function parameter type to accept a Person object.

Typescript

function greet(person: [1]) { console.log(`Hello, ${person.name}`); }

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

Astring

Bany

Cnumber

DPerson

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' which causes an error accessing 'name'.

Using 'number' or 'any' which are not specific types.

The function expects a Person type to access name property safely.

4fill in blank

hard

Fill both blanks to create a function that returns the person's age.

Typescript

function getAge([1]: Person): [2] { return [1].age; }

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

Aperson

Bnumber

Cstring

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' as return type instead of 'number'.

Using 'age' as parameter name without type.

The function takes a parameter named person of type Person and returns a number representing the age.

5fill in blank

hard

Fill all three blanks to create a typed object and access its properties.

Typescript

type [1] = { [2]: string; [3]: number };

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

APerson

Bname

Cage

DUser

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'User' instead of 'Person' as type name.

Mixing up property names or types.

The type is named Person with properties name (string) and age (number).

Practice

1. Why do we need object types in TypeScript?

easy

A. To create HTML elements automatically

B. To make variables hold only numbers

C. To describe the shape of data with named properties

D. To run JavaScript code faster

Solution

1. Step 1: Understand what object types represent

   Object types describe how data is structured by naming its properties and their types.

2. Step 2: Recognize the purpose of object types

   They help ensure that data used in code matches the expected shape, preventing mistakes.

3. Final Answer:

   To describe the shape of data with named properties -> Option C

4. Quick Check:

   Object types = describe data shape [OK]

Hint: Object types define data shape with property names [OK]

Common Mistakes:

• Thinking object types only hold numbers
• Confusing object types with performance tools
• Believing object types create HTML elements

2. Which of the following is the correct syntax to define an object type in TypeScript?

easy

A. let Person = (name: string, age: number);

B. type Person = { name: string; age: number; };

C. function Person(name: string, age: number) {}

D. const Person = [name: string, age: number];

Solution

1. Step 1: Identify object type syntax

   In TypeScript, object types are defined using the type keyword followed by property names and types inside curly braces.

2. Step 2: Compare options

   type Person = { name: string; age: number; }; correctly uses type Person = { name: string; age: number; };. Others are function or array syntax, not object types.

3. Final Answer:

   type Person = { name: string; age: number; }; -> Option B

4. Quick Check:

   Object type syntax = type + { properties } [OK]

Hint: Object types use 'type' and curly braces with properties [OK]

Common Mistakes:

• Using function syntax instead of type
• Using array syntax for object types
• Missing curly braces around properties

3. What will be the result of this TypeScript code?

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

medium

A. 2020

B. Toyota

C. undefined

D. Error: Property 'year' does not exist

Solution

1. Step 1: Understand the object type and assignment

   The type Car expects properties make (string) and year (number). The object assigned matches this.

2. Step 2: Check the console.log output

   Logging myCar.year prints the number 2020, the value assigned to year.

3. Final Answer:

   2020 -> Option A

4. Quick Check:

   Accessing property year = 2020 [OK]

Hint: Access object property to get its value [OK]

Common Mistakes:

• Confusing property names and values
• Expecting string instead of number
• Thinking property does not exist

4. Identify the error in this TypeScript code:

type User = { username: string; age: number };
const user: User = { username: "Anna" };

medium

A. Missing property 'age' in object assignment

B. Wrong type for 'username', should be number

C. Extra property 'username' not allowed

D. Syntax error in type declaration

Solution

1. Step 1: Check object properties against type

   The type User requires both username and age properties.

2. Step 2: Find missing or incorrect properties

   The object only has username, missing age, causing a TypeScript error.

3. Final Answer:

   Missing property 'age' in object assignment -> Option A

4. Quick Check:

   All required properties must be present [OK]

Hint: Check all required properties are included [OK]

Common Mistakes:

• Ignoring missing properties
• Assuming extra properties cause error
• Confusing property types

5. You want to create a function that accepts an object with properties title (string) and optional year (number). Which object type correctly describes this?

hard

A. type Movie = { title: string; year: number; };

B. type Movie = { title?: string; year: number; };

C. type Movie = { title: string | number; year?: string; };

D. type Movie = { title: string; year?: number; };

Solution

1. Step 1: Understand optional properties in object types

   Optional properties are marked with a question mark ? after the property name.

2. Step 2: Match the requirement

   The property title is required (string), and year is optional (number). type Movie = { title: string; year?: number; }; matches this exactly.

3. Final Answer:

   type Movie = { title: string; year?: number; }; -> Option D

4. Quick Check:

   Optional property uses '?' [OK]

Hint: Use '?' to mark optional properties in object types [OK]

Common Mistakes:

• Marking required properties as optional
• Wrong property types
• Missing '?' for optional properties