Capitalize and Uncapitalize types in Typescript - Step-by-Step Execution

Concept Flow - Capitalize and Uncapitalize types

Start with string type

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Apply Capitalize<T>

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First letter uppercase

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Resulting type

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Start with string type

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Apply Uncapitalize<T>

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First letter lowercase

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Resulting type

Capitalize<T> makes the first letter uppercase; Uncapitalize<T> makes the first letter lowercase of a string type.

Execution Sample

Typescript

type Cap = Capitalize<'hello'>;
type Uncap = Uncapitalize<'WORLD'>;

This code creates two types: one with first letter capitalized, one with first letter uncapitalized.

Execution Table

Step	Type Operation	Input Type	Result Type
1	Capitalize<'hello'>	'hello'	'Hello'
2	Uncapitalize<'WORLD'>	'WORLD'	'wORLD'
3	Capitalize<'typescript'>	'typescript'	'Typescript'
4	Uncapitalize<'TypeScript'>	'TypeScript'	'typeScript'
5	Capitalize<'123abc'>	'123abc'	'123abc' (unchanged, no letter)
6	Uncapitalize<'ABC123'>	'ABC123'	'aBC123'
Exit	No more operations	-	-

💡 All examples processed, showing how first letter changes or stays if not a letter.

Variable Tracker

Type Alias	Initial Input	After Operation	Final Result
Cap	'hello'	Capitalize applied	'Hello'
Uncap	'WORLD'	Uncapitalize applied	'wORLD'

Key Moments - 2 Insights

Why does Capitalize<'123abc'> not change the string?

Because Capitalize only changes the first letter if it is a letter; digits stay the same as shown in execution_table row 5.

Does Uncapitalize change all letters or just the first one?

Only the first letter is changed to lowercase, the rest remain unchanged, as seen in execution_table rows 2 and 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the result of Capitalize<'hello'> at step 1?

A 'hello'

B 'HELLO'

C 'Hello'

D 'hELLO'

💡 Hint

Check the Result Type column at step 1 in the execution_table.

At which step does Uncapitalize change 'WORLD' to 'wORLD'?

A Step 2

B Step 5

C Step 3

D Step 4

💡 Hint

Look at the Type Operation and Result Type columns in execution_table.

If you apply Capitalize to 'typescript', what will be the result?

A 'typescript'

B 'Typescript'

C 'TYPESCRIPT'

D 'tYPESCRIPT'

💡 Hint

Refer to step 3 in execution_table for Capitalize<'typescript'>.

Concept Snapshot

Capitalize<T>: changes first letter of string type T to uppercase.
Uncapitalize<T>: changes first letter of string type T to lowercase.
Only affects first character if it is a letter.
Useful for transforming string literal types.
Syntax: type NewType = Capitalize<'string'> or Uncapitalize<'STRING'>.

Full Transcript

This visual execution shows how TypeScript's Capitalize and Uncapitalize types work. Capitalize takes a string type and makes its first letter uppercase, while Uncapitalize makes the first letter lowercase. For example, Capitalize<'hello'> becomes 'Hello', and Uncapitalize<'WORLD'> becomes 'wORLD'. If the first character is not a letter, like a digit, Capitalize does not change it. Only the first letter is affected; the rest of the string stays the same. This helps when you want to transform string literal types in your code. The execution table traces each step with input and output types, and the variable tracker shows how type aliases change. The quiz tests understanding of these transformations.