Capitalize and Uncapitalize types in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to capitalize the first letter of the string type.

Typescript

type Capitalized = [1]<'hello'>;

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

AUncapitalize

BUppercase

CCapitalize

DLowercase

Attempts:

3 left

💡 Hint

Common Mistakes

Using Uncapitalize instead of Capitalize.

Using Uppercase which changes all letters, not just the first.

The Capitalize utility type changes the first letter of a string type to uppercase.

2fill in blank

medium

Complete the code to uncapitalize the first letter of the string type.

Typescript

type LowercaseFirst = [1]<'Hello'>;

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

ACapitalize

BLowercase

CUppercase

DUncapitalize

Attempts:

3 left

💡 Hint

Common Mistakes

Using Capitalize instead of Uncapitalize.

Using Lowercase which changes all letters, not just the first.

The Uncapitalize utility type changes the first letter of a string type to lowercase.

3fill in blank

hard

Fix the error in the code to capitalize the first letter of the string type.

Typescript

type FixCap = [1]<'world'>;

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

ACapitalize

Bcapitalize

CUppercase

Duncapitalize

Attempts:

3 left

💡 Hint

Common Mistakes

Writing 'capitalize' with a lowercase c causes an error.

Using 'uncapitalize' which is not a valid utility type.

TypeScript utility types are case-sensitive. The correct type is Capitalize with a capital C.

4fill in blank

hard

Fill both blanks to create a type that capitalizes and then uncapitalizes a string.

Typescript

type Transform = [1]<[2]<'example'>>;

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

ACapitalize

BUncapitalize

Attempts:

3 left

💡 Hint

Common Mistakes

Reversing the order of Capitalize and Uncapitalize.

Using the same utility type twice.

This type first uncapitalizes the string, then capitalizes it, effectively normalizing the first letter.

5fill in blank

hard

Fill all three blanks to create a type that uncapitalizes, capitalizes, and then uncapitalizes a string.

Typescript

type ComplexTransform = [1]<[2]<[3]<'Test'>>>;

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

AUncapitalize

BCapitalize

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order of utility types.

Using lowercase names for utility types.

This type applies Capitalize, then Capitalize again, then Uncapitalize, showing nested transformations.

Practice

1. What does the Capitalize<T> utility type do in TypeScript?

easy

A. It makes the first letter of a string type lowercase.

B. It makes the entire string type uppercase.

C. It reverses the string type.

D. It makes the first letter of a string type uppercase.

Solution

1. Step 1: Understand the purpose of Capitalize<T>

   The Capitalize utility type changes only the first letter of a string type to uppercase.

2. Step 2: Compare with other options

   Other options describe different transformations like full uppercase or lowercase, which are not what Capitalize does.

3. Final Answer:

   It makes the first letter of a string type uppercase. -> Option D

4. Quick Check:

   Capitalize = first letter uppercase [OK]

Hint: Capitalize only changes the first letter to uppercase [OK]

Common Mistakes:

• Thinking it changes the whole string to uppercase
• Confusing Capitalize with Uncapitalize
• Assuming it reverses the string

2. Which of the following is the correct syntax to uncapitalize a string type T in TypeScript?

easy

A. Uncapitalize<T>

B. Uncapitalize[T]

C. uncapitalize<T>

D. Uncapitalize(T)

Solution

1. Step 1: Recall TypeScript generic syntax

   Generic utility types use angle brackets <> to specify the type parameter, like Uncapitalize<T>.

2. Step 2: Check case sensitivity and brackets

   TypeScript is case sensitive and uses angle brackets, so 'Uncapitalize<T>' is correct, while parentheses or square brackets are invalid here.

3. Final Answer:

   Uncapitalize<T> -> Option A

4. Quick Check:

   Generic types use <> brackets [OK]

Hint: Use angle brackets for generic types like Uncapitalize<T> [OK]

Common Mistakes:

• Using parentheses instead of angle brackets
• Writing utility type names in lowercase
• Using square brackets instead of angle brackets

3. What is the resulting type of Capitalize<'hello world'> in TypeScript?

medium

A. 'Hello world'

B. 'hello world'

C. 'HELLO WORLD'

D. 'hELLO WORLD'

Solution

1. Step 1: Apply Capitalize to the string literal

   Capitalize changes only the first letter to uppercase, so 'hello world' becomes 'Hello world'.

2. Step 2: Confirm the rest of the string remains unchanged

   Only the first character changes; the rest stays lowercase as in the original string.

3. Final Answer:

   'Hello world' -> Option A

4. Quick Check:

   Capitalize<'hello world'> = 'Hello world' [OK]

Hint: Only first letter changes to uppercase, rest stays same [OK]

Common Mistakes:

• Making the entire string uppercase
• Not changing any letter
• Changing only the first letter to lowercase

4. Given the code snippet:

type T = uncapitalize<'HelloWorld'>;

What is the error in this code?

medium

A. The string literal should be lowercase.

B. The utility type name should be capitalized as Uncapitalize.

C. The angle brackets should be replaced with parentheses.

D. There is no error; the code is correct.

Solution

1. Step 1: Check utility type name casing

   TypeScript utility types like Uncapitalize must start with uppercase letters; 'uncapitalize' is invalid.

2. Step 2: Verify other syntax elements

   The string literal and angle brackets are correct; only the utility type name casing is wrong.

3. Final Answer:

   The utility type name should be capitalized as Uncapitalize. -> Option B

4. Quick Check:

   Utility types start with uppercase letters [OK]

Hint: Utility types always start with uppercase letters [OK]

Common Mistakes:

• Using lowercase for utility type names
• Changing brackets to parentheses
• Thinking string literal case matters here

5. How can you create a new type that capitalizes the first letter of each word in a union type 'apple' | 'banana' | 'cherry' using TypeScript's Capitalize utility?

hard

A. type Fruits = Capitalize<'apple banana cherry'>;

B. type Fruits = Capitalize<apple | banana | cherry>;

C. type Fruits = Capitalize<'apple'> | Capitalize<'banana'> | Capitalize<'cherry'>;

D. type Fruits = Uncapitalize<'apple' | 'banana' | 'cherry'>;

Solution

1. Step 1: Understand Capitalize on union types

   Capitalizing a union type like 'apple' | 'banana' | 'cherry' directly does not capitalize each word separately.

2. Step 2: Capitalize each string literal individually

   Applying Capitalize to each string separately and then unioning them ensures each word's first letter is capitalized.

3. Final Answer:

   type Fruits = Capitalize<'apple'> | Capitalize<'banana'> | Capitalize<'cherry'>; -> Option C

4. Quick Check:

   Capitalize each union member separately [OK]

Hint: Capitalize each union member separately for correct result [OK]

Common Mistakes:

• Applying Capitalize to the whole union at once
• Using Uncapitalize instead of Capitalize
• Trying to capitalize a single string with all words