Template literal types in Typescript - Mini Project: Build & Apply

Using Template Literal Types in TypeScript

📖 Scenario: You are building a small TypeScript program to create specific string types that combine fixed words with variable parts. This helps catch mistakes early by only allowing certain string patterns.

🎯 Goal: Learn how to use template literal types in TypeScript to create new string types by combining fixed and variable parts.

📋 What You'll Learn

Create a union type of colors

Create a union type of sizes

Create a template literal type combining colors and sizes

Declare a variable using the template literal type

Assign a valid value to the variable

Print the variable value

💡 Why This Matters

🌍 Real World

Template literal types help developers create precise string types that match specific patterns, reducing bugs caused by typos or invalid strings.

💼 Career

Many TypeScript projects use template literal types for safer code, especially in UI development, API design, and configuration management.

1

Create union types for colors and sizes

Create a union type called Color with the values 'red', 'green', and 'blue'. Also create a union type called Size with the values 'small', 'medium', and 'large'.

Typescript

# Create the union type Color and Size
// Your code here

Hint

Use the | symbol to combine string literals into a union type.

2

Create a template literal type combining Color and Size

Create a template literal type called ColoredSize that combines Color and Size with a dash - between them. Use the syntax `${Color}-${Size}`.

Typescript

type Color = 'red' | 'green' | 'blue';
type Size = 'small' | 'medium' | 'large';

// Create the template literal type ColoredSize
// Your code here

Hint

Use backticks and ${} to create the template literal type.

3

Declare a variable using the template literal type

Declare a variable called item of type ColoredSize. Assign it the value 'green-medium'.

Typescript

type Color = 'red' | 'green' | 'blue';
type Size = 'small' | 'medium' | 'large';

type ColoredSize = `${Color}-${Size}`;

// Declare variable item of type ColoredSize and assign 'green-medium'
// Your code here

Hint

Use let to declare the variable and specify the type after the colon.

4

Print the variable value

Write a console.log statement to print the value of the variable item.

Typescript

type Color = 'red' | 'green' | 'blue';
type Size = 'small' | 'medium' | 'large';

type ColoredSize = `${Color}-${Size}`;

let item: ColoredSize = 'green-medium';

// Print the value of item
// Your code here

Hint

Use console.log(item); to print the value.

Practice

1. What is the main purpose of template literal types in TypeScript?

easy

A. To create new string types by combining existing string types

B. To perform string concatenation at runtime

C. To define numeric ranges for variables

D. To declare functions with string parameters

Solution

1. Step 1: Understand template literal types

   Template literal types allow combining string literal types to form new string types.

2. Step 2: Differentiate from runtime string operations

   They are used at compile time for type checking, not for runtime string concatenation or numeric ranges.

3. Final Answer:

   To create new string types by combining existing string types -> Option A

4. Quick Check:

   Template literal types build new string types [OK]

Hint: Remember: template literal types create types, not runtime strings [OK]

Common Mistakes:

• Confusing template literal types with runtime string concatenation
• Thinking they define numeric ranges
• Assuming they declare functions

2. Which of the following is the correct syntax to define a template literal type combining 'hello' and 'world' with a space in between?

easy

A. type Greeting = `${'hello'} ${'world'}`;

B. type Greeting = `hello world`;

C. type Greeting = `${'hello' + ' ' + 'world'}`;

D. type Greeting = '${hello} ${world}';

Solution

1. Step 1: Understand template literal syntax

   Template literal types use backticks and ${} to embed string literals or types.

2. Step 2: Check each option

   type Greeting = `${'hello'} ${'world'}`; correctly uses backticks and embeds 'hello' and 'world' with a space between them. type Greeting = `${'hello' + ' ' + 'world'}`; uses + inside template literal which is invalid. type Greeting = `hello world`; is a plain string literal, not combining types. type Greeting = '${hello} ${world}'; uses single quotes and incorrect syntax.

3. Final Answer:

   type Greeting = `${'hello'} ${'world'}`; -> Option A

4. Quick Check:

   Correct template literal syntax uses backticks and ${} [OK]

Hint: Use backticks and ${} to embed strings in template literal types [OK]

Common Mistakes:

• Using + inside template literals
• Using single quotes instead of backticks
• Writing plain strings instead of template literal types

3. Given the following TypeScript code, what is the type of greeting?

type Hello = 'hello';
type World = 'world';
type Greeting = `${Hello} ${World}`;

const greeting: Greeting = 'hello world';

medium

A. `${Hello}${World}`

B. 'hello' | 'world'

C. string

D. 'hello world'

Solution

1. Step 1: Understand the template literal type definition

   The type Greeting combines 'hello' and 'world' with a space, so it represents the exact string 'hello world'.

2. Step 2: Check the assigned value type

   The variable greeting is assigned 'hello world', which matches the Greeting type exactly.

3. Final Answer:

   'hello world' -> Option D

4. Quick Check:

   Template literal type matches exact combined string [OK]

Hint: Template literal types produce exact combined string types [OK]

Common Mistakes:

• Thinking the type is a union of 'hello' and 'world'
• Assuming it is a generic string
• Confusing the template literal syntax with a string literal

4. Identify the error in the following TypeScript code using template literal types:

type Prefix = 'pre';
type Suffix = 'fix';
type Combined = `${Prefix + Suffix}`;

const word: Combined = 'prefix';

medium

A. Missing backticks around the template literal

B. The variable word has wrong value assigned

C. Using + operator inside template literal types is invalid

D. Prefix and Suffix types must be numbers

Solution

1. Step 1: Check template literal syntax

   Inside template literal types, you cannot use the + operator to concatenate types; you embed types directly with ${}.

2. Step 2: Identify the error

   The expression `${Prefix + Suffix}` is invalid syntax. It should be `${Prefix}${Suffix}` without +.

3. Final Answer:

   Using + operator inside template literal types is invalid -> Option C

4. Quick Check:

   Template literal types concatenate by adjacent ${} without + [OK]

Hint: No + inside template literal types; just place ${} side by side [OK]

Common Mistakes:

• Using + operator inside template literal types
• Confusing runtime string concatenation with type syntax
• Assuming types must be numbers to concatenate

5. You want to create a type EventName that represents strings like 'click_button' or 'hover_link' by combining event types 'click' | 'hover' and target types 'button' | 'link' separated by an underscore. Which of the following correctly defines EventName?

hard

A. type EventName = `${click | hover}_${button | link}`;

B. type EventName = `${'click' | 'hover'}_${'button' | 'link'}`;

C. type EventName = `${'click' & 'hover'}_${'button' & 'link'}`;

D. type EventName = '${click | hover}_${button | link}';

Solution

1. Step 1: Understand union types in template literals

   Using union types inside ${} creates all combinations of the union members.

2. Step 2: Check syntax correctness

   type EventName = `${'click' | 'hover'}_${'button' | 'link'}`; correctly uses backticks and unions inside ${} with an underscore between them. type EventName = '${click | hover}_${button | link}'; uses single quotes and invalid syntax. type EventName = `${'click' & 'hover'}_${'button' & 'link'}`; uses & which means intersection, not union, so it won't produce combinations. type EventName = `${click | hover}_${button | link}`; misses quotes around string literals.

3. Final Answer:

   type EventName = `${'click' | 'hover'}_${'button' | 'link'}`; -> Option B

4. Quick Check:

   Union types inside template literals create all string combinations [OK]

Hint: Use unions inside ${} with backticks to create all string combos [OK]

Common Mistakes:

• Using & instead of | for unions
• Omitting quotes around string literals
• Using single quotes instead of backticks