Capitalize and Uncapitalize types in Typescript - Mini Project: Build & Apply

Capitalize and Uncapitalize Types in TypeScript

📖 Scenario: Imagine you are working on a user interface where you need to format strings to show names properly. Sometimes you want the first letter capitalized, and other times you want it lowercase.

🎯 Goal: You will create TypeScript types that change the first letter of a string type to uppercase or lowercase using Capitalize and Uncapitalize utility types.

📋 What You'll Learn

Create a string type called originalName with the value 'helloWorld'.

Create a type called capitalizedName that uses Capitalize on originalName.

Create a type called uncapitalizedName that uses Uncapitalize on capitalizedName.

Print the values of capitalizedName and uncapitalizedName as strings.

💡 Why This Matters

🌍 Real World

Formatting user names, titles, or labels in user interfaces to follow style guides.

💼 Career

Understanding TypeScript utility types helps write safer and cleaner code in frontend and backend projects.

1

Create the original string type

Create a string type called originalName with the exact value 'helloWorld'.

Typescript

# Your code here
// Create a type called originalName with value 'helloWorld'

Hint

Use type originalName = 'helloWorld'; to create the string type.

2

Create a capitalized type

Create a type called capitalizedName that uses Capitalize on originalName.

Typescript

type originalName = 'helloWorld';
# Your code here
// Create capitalizedName using Capitalize on originalName

Hint

Use type capitalizedName = Capitalize; to capitalize the first letter.

3

Create an uncapitalized type

Create a type called uncapitalizedName that uses Uncapitalize on capitalizedName.

Typescript

type originalName = 'helloWorld';
type capitalizedName = Capitalize;
# Your code here
// Create uncapitalizedName using Uncapitalize on capitalizedName

Hint

Use type uncapitalizedName = Uncapitalize; to uncapitalize the first letter.

4

Print the capitalized and uncapitalized types

Write two console.log statements to print the values of capitalizedName and uncapitalizedName as strings. Use type assertions to convert types to strings.

Typescript

type originalName = 'helloWorld';
type capitalizedName = Capitalize;
type uncapitalizedName = Uncapitalize;

// Your code here
// Print capitalizedName and uncapitalizedName as strings

Hint

Assign the string values to constants with the correct types, then print them.

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