Template literal types in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a template literal type that combines 'hello' and a string.

Typescript

type Greeting = `hello [1]`;

Drag options to blanks, or click blank then click option'

Astring

Bnumber

Cboolean

Dworld

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'world' limits the type to only 'helloworld'.

Using 'number' or 'boolean' causes type errors.

The template literal type uses string to allow any string after 'hello'.

2fill in blank

medium

Complete the code to create a template literal type that prefixes 'id_' to a number.

Typescript

type ID = `id_[1]`;

Drag options to blanks, or click blank then click option'

Anumber

Bstring

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' allows any string, not just numbers.

Using 'boolean' or 'any' is not specific enough.

The template literal type uses number to allow numeric suffixes after 'id_'.

3fill in blank

hard

Fix the error in the template literal type that combines 'user_' with a union of 'admin' or 'guest'.

Typescript

type UserRole = `user_[1]`;

Drag options to blanks, or click blank then click option'

Aadmin | guest

B'admin' | 'guest'

C"admin" | "guest"

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting quotes causes syntax errors.

Using plain identifiers without quotes is invalid.

Template literal types require string literal types inside, so quotes are needed around 'admin' and 'guest'.

4fill in blank

hard

Fill both blanks to create a template literal type that combines a color and a size with a dash between.

Typescript

type Product = `$[1]-[2]`;

Drag options to blanks, or click blank then click option'

A'red' | 'blue' | 'green'

B'small' | 'medium' | 'large'

Cstring

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' or 'number' allows any value, not specific ones.

Mixing up the order of color and size.

The first blank is a union of color strings, the second blank is a union of size strings, combined with a dash.

5fill in blank

hard

Fill all three blanks to create a template literal type for event names with prefix, action, and suffix.

Typescript

type EventName = `$[1]_$[2]_$[3]`;

Drag options to blanks, or click blank then click option'

A'click' | 'hover' | 'scroll'

B'start' | 'end' | 'move'

C'button' | 'link' | 'image'

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using unions in the wrong order.

Not using underscores between parts.

The prefix is any string, followed by an action from the first union, then a suffix from the second union, all joined by underscores.

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