Object type annotation inline in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an object with inline type annotation.

Typescript

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

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

A{ name: string; age: number }

B[string, number]

Cstring

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using array syntax instead of object syntax for the type.

Using primitive types like string or number instead of an object type.

The inline type annotation for an object with properties name (string) and age (number) is { name: string; age: number }.

2fill in blank

medium

Complete the code to annotate an object with optional property.

Typescript

const product: [1] = { id: 101, name: "Book" };

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

A{ id: number; name?: string; price: number }

B{ id: number; name: string; price: number }

C{ id: number; name: string; price?: number }

D{ id?: number; name: string; price: number }

Attempts:

3 left

💡 Hint

Common Mistakes

Marking required properties as optional.

Omitting the question mark for optional properties.

The property price is optional, so it is marked with a question mark (price?: number).

3fill in blank

hard

Fix the error in the inline object type annotation.

Typescript

const settings: [1] = { darkMode: true, fontSize: 14 };

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

A{ darkMode: boolean, fontSize: number }

B{ darkMode: bool; fontSize: int }

C{ darkMode: Boolean; fontSize: Number }

D{ darkMode: boolean; fontSize: number }

Attempts:

3 left

💡 Hint

Common Mistakes

Using commas instead of semicolons between properties.

Using capitalized Boolean or Number types.

In TypeScript, object properties in type annotations are separated by semicolons, not commas.

4fill in blank

hard

Complete the code to declare an object with a nested object type inline.

Typescript

const employee: [1] = { name: "John", address: { city: "NY", zip: 10001 } };

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

A{ name: string, address: { city: string; zip: number } }

B{ name: string; address: { city: string; zip: number } }

C{ name: string; address: { city: string, zip: number } }

D{ name: string, address: { city: string, zip: number } }

Attempts:

3 left

💡 Hint

Common Mistakes

Using commas instead of semicolons in object type annotations.

Mixing commas and semicolons inconsistently.

Use semicolons to separate properties in both the outer and nested object types.

5fill in blank

hard

Fill all three blanks to declare an object with inline type annotation including a method.

Typescript

const calculator: [1] = { add: (a: number, b: number) => a [2] b, subtract: (a: number, b: number) => a [3] b };

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

A{ add: (x: number, y: number) => number; subtract: (x: number, y: number) => number }

B+

C-

D{ add: (a: number, b: number) => number; subtract: (a: number, b: number) => number }

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names in the type annotation.

Using wrong operators in the method implementations.

Mixing commas and semicolons in type annotations.

The object type annotation declares methods add and subtract with correct parameter names and return types. The add method uses + and subtract uses - operators.

Practice

1. What does inline object type annotation in TypeScript do?
let user: { name: string; age: number };

easy

A. It defines the shape of the object directly where it is used.

B. It creates a new class named user.

C. It converts the object to a JSON string.

D. It declares a function that returns an object.

Solution

1. Step 1: Understand inline object type annotation

   Inline object type annotation specifies the properties and their types directly in the variable declaration.

2. Step 2: Analyze the example

   The example { name: string; age: number } tells TypeScript that user must have a string name and a number age.

3. Final Answer:

   It defines the shape of the object directly where it is used. -> Option A

4. Quick Check:

   Inline type annotation = direct object shape [OK]

Hint: Inline means type right where variable is declared [OK]

Common Mistakes:

• Thinking it creates a class
• Confusing with JSON conversion
• Assuming it declares a function

2. Which of the following is the correct syntax for inline object type annotation in TypeScript?

easy

A. let data: (id: number, title: string) = { id: 1, title: 'Book' };

B. let data = { id: number, title: string } = { id: 1, title: 'Book' };

C. let data: { id: number; title: string } = { id: 1, title: 'Book' };

D. let data: [number, string] = { id: 1, title: 'Book' };

Solution

1. Step 1: Identify correct inline object type syntax

   Inline object type annotation uses curly braces with property names and types, like { id: number; title: string }.

2. Step 2: Check each option

   let data: { id: number; title: string } = { id: 1, title: 'Book' }; correctly uses this syntax and assigns a matching object. Options A, B, and D use incorrect syntax or types.

3. Final Answer:

   let data: { id: number; title: string } = { id: 1, title: 'Book' }; -> Option C

4. Quick Check:

   Curly braces with property types = correct syntax [OK]

Hint: Use curly braces with property: type pairs [OK]

Common Mistakes:

• Using parentheses instead of braces
• Assigning types inside object literal
• Confusing arrays with objects

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

function printUser(user: { name: string; age: number }) {
  console.log(`${user.name} is ${user.age} years old.`);
}
printUser({ name: 'Alice', age: 30 });

medium

A. Alice is 30 years old.

B. Error: Missing property age

C. Alice is undefined years old.

D. Error: Type mismatch in function argument

Solution

1. Step 1: Understand the function parameter type

   The function expects an object with name as string and age as number.

2. Step 2: Check the argument passed

   The argument { name: 'Alice', age: 30 } matches the expected type exactly.

3. Final Answer:

   Alice is 30 years old. -> Option A

4. Quick Check:

   Matching object type = correct output [OK]

Hint: Match object shape exactly to avoid errors [OK]

Common Mistakes:

• Passing missing or extra properties
• Using wrong types for properties
• Ignoring inline type annotation

4. Identify the error in this TypeScript code snippet:

let product: { id: number; name: string } = { id: 101, name: 'Pen', price: 5 };

medium

A. Missing semicolon after object declaration.

B. Extra property 'price' not allowed in inline type annotation.

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

D. Object must be assigned to a function, not a variable.

Solution

1. Step 1: Check the declared type

   The type annotation allows only id and name properties.

2. Step 2: Compare with assigned object

   The object has an extra property price which is not declared in the type, causing an error.

3. Final Answer:

   Extra property 'price' not allowed in inline type annotation. -> Option B

4. Quick Check:

   Extra properties cause errors in inline types [OK]

Hint: Only use declared properties in inline object types [OK]

Common Mistakes:

• Adding extra properties not in type
• Ignoring TypeScript error messages
• Confusing semicolon placement

5. You want to write a function that accepts an object with a string title and an optional number year. Which inline type annotation is correct?

hard

A. function showBook(book: { title: string; year: number | undefined }) { console.log(book.title); }

B. function showBook(book: { title: string | number; year: number }) { console.log(book.title); }

C. function showBook(book: { title?: string; year: number }) { console.log(book.title); }

D. function showBook(book: { title: string; year?: number }) { console.log(book.title); }

Solution

1. Step 1: Understand optional properties in inline types

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

2. Step 2: Analyze each option

   function showBook(book: { title: string; year?: number }) { console.log(book.title); } correctly marks year as optional with year?: number. Other options either make title optional or use incorrect types.

3. Final Answer:

   function showBook(book: { title: string; year?: number }) { console.log(book.title); } -> Option D

4. Quick Check:

   Optional property uses ? after name [OK]

Hint: Use ? after property name for optional fields [OK]

Common Mistakes:

• Making required properties optional
• Using union types incorrectly
• Confusing optional with undefined type