Template literal types in Typescript

Introduction

Template literal types let you create new string types by combining other string types. This helps catch mistakes and make your code clearer.

When you want to build specific string patterns from smaller parts.

When you want TypeScript to check if a string matches a certain format.

When you want to create flexible but safe string types for things like URLs, CSS classes, or event names.

Syntax

Typescript

type NewType = `${Prefix}${Middle}${Suffix}`;

You use backticks (`) around the template, like in JavaScript template strings.

Each part inside ${} must be a string literal type or a union of string literals.

Examples

This creates a type that can be either "Hello, World!" or "Hello, Friend!".

Typescript

type Greeting = `Hello, ${"World" | "Friend"}!`;

This type can be "onClick", "onHover", "offClick", or "offHover".

Typescript

type EventName = `${"on" | "off"}${"Click" | "Hover"}`;

This type represents CSS class names like "btn-primary", "btn-secondary", or "btn-danger".

Typescript

type CssClass = `btn-${"primary" | "secondary" | "danger"}`;

Sample Program

This program defines a type Status with three possible values. Then it creates a template literal type Message that combines a fixed string with Status. The function showMessage only accepts strings matching the Message type. Trying to pass a wrong string causes an error.

Typescript

type Status = "success" | "error" | "loading";

type Message = `Status is: ${Status}`;

function showMessage(msg: Message) {
  console.log(msg);
}

showMessage("Status is: success");
// showMessage("Status is: done"); // Error: Argument of type '"Status is: done"' is not assignable to parameter of type 'Message'.

Important Notes

Template literal types only work with string literal types, not general strings.

They help catch errors early by limiting allowed string values.

You can combine them with unions to create many possible string patterns.

Summary

Template literal types build new string types by combining smaller string types.

They help TypeScript check if strings match specific patterns.

Use them to make your code safer and clearer when working with strings.

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