Capitalize and Uncapitalize types in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Capitalize type example?

Consider the following TypeScript code using the Capitalize utility type. What is the type of Result?

Typescript

type Result = Capitalize<'hello world'>;

// What is the type of Result?

A "hello world"

B "Hello World"

C "Hello world"

D "HELLO WORLD"

💡 Hint

Capitalize only changes the first character to uppercase, leaving the rest unchanged.

The Capitalize type changes only the first character of the string literal type to uppercase. So 'hello world' becomes 'Hello world'.

❓ Predict Output

intermediate

What is the output of this Uncapitalize type example?

Look at this TypeScript code using the Uncapitalize utility type. What is the type of Result?

Typescript

type Result = Uncapitalize<'Hello World'>;

// What is the type of Result?

A "hello World"

B "hello world"

C "Hello World"

D "HELLO WORLD"

💡 Hint

Uncapitalize only changes the first character to lowercase, leaving the rest unchanged.

The Uncapitalize type changes only the first character of the string literal type to lowercase. So 'Hello World' becomes 'hello World'.

🧠 Conceptual

advanced

Which option correctly describes Capitalize and Uncapitalize behavior?

Which statement correctly describes how the TypeScript utility types Capitalize and Uncapitalize work?

A Capitalize changes the first character to uppercase and all others to lowercase; Uncapitalize changes the first character to lowercase and all others to uppercase.

B Capitalize changes all characters to uppercase; Uncapitalize changes all characters to lowercase.

C Capitalize and Uncapitalize only work on number types, not strings.

D Capitalize changes the first character to uppercase; Uncapitalize changes the first character to lowercase; both leave the rest of the string unchanged.

💡 Hint

Think about how only the first letter is affected.

Capitalize and Uncapitalize only affect the first character of a string literal type. They do not change the rest of the string.

❓ Predict Output

advanced

What is the type of Result after nested Capitalize and Uncapitalize?

Given this TypeScript code, what is the type of Result?

Typescript

type Result = Uncapitalize<Capitalize<'tYPEScript'>>;

// What is Result?

A "typeScript"

B "tYPEScript"

C "TypeScript"

D "TYPEScript"

💡 Hint

Apply Capitalize first, then Uncapitalize on the result.

Capitalize<'tYPEScript'> makes the first letter uppercase: 'TYPEScript'. Then Uncapitalize<'TYPEScript'> makes the first letter lowercase again: 'tYPEScript'.

❓ Predict Output

expert

What is the output of this mapped type using Capitalize and Uncapitalize?

Consider this TypeScript code that uses mapped types with Capitalize and Uncapitalize. What is the resulting type of NewObj?

Typescript

type Original = {
  firstName: string;
  lastName: string;
  Age: number;
};

type NewObj = {
  [K in keyof Original as Capitalize<Uncapitalize<string & K>>]: Original[K]
};

A { FirstName: string; LastName: string; Age: number; }

B { Firstname: string; Lastname: string; Age: number; }

C { firstname: string; lastname: string; age: number; }

D { firstName: string; lastName: string; Age: number; }

💡 Hint

Uncapitalize then Capitalize on the key name affects only the first letter casing.

For keys 'firstName' and 'lastName', Uncapitalize does nothing (first letter already lowercase), then Capitalize makes first letter uppercase while leaving the rest unchanged: 'FirstName', 'LastName'. For 'Age', Uncapitalize makes 'age', then Capitalize makes 'Age'.