Template literal types in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Template Literal Type Concatenation

What is the type of Result after this code runs?

Typescript

type A = "Hello";
type B = "World";
type Result = `${A} ${B}!`;

// What is Result?

A string

B "HelloWorld!"

C "Hello World"

D "Hello World!"

💡 Hint

Template literal types combine string literal types exactly as written.

The template literal type `${A} ${B}!` concatenates the literal types A and B with a space and exclamation mark, resulting in the exact string literal type "Hello World!".

❓ Predict Output

intermediate

Template Literal Type with Union Types

What is the type of Greeting after this code?

Typescript

type Name = "Alice" | "Bob";
type Greeting = `Hello, ${Name}!`;

// What is Greeting?

A "Hello, Alice!" | "Hello, Bob!"

B "Hello, Alice" | "Hello, Bob"

C "Hello, Alice!" & "Hello, Bob!"

D string

💡 Hint

Template literal types distribute over unions.

When a template literal type uses a union type inside, it creates a union of all possible combinations. Here, Greeting is the union of "Hello, Alice!" and "Hello, Bob!".

🔧 Debug

advanced

Why does this Template Literal Type cause an error?

This code causes a TypeScript error. What is the reason?

Typescript

type Prefix = "pre";
type Suffix = number;
type Combined = `${Prefix}${Suffix}`;

// Why is this an error?

A Template literal types cannot concatenate two types.

B Suffix is the number type, template literal types only accept string literal types or numeric literal types.

C Prefix must be a number, not a string.

D Combined must be declared as a string, not a template literal type.

💡 Hint

Check the allowed types inside template literal types.

Template literal types accept string literal types or numeric literal types, but the type Suffix is the number type, not a string literal type or numeric literal type. It must be a string literal type or a numeric literal type.

🧠 Conceptual

advanced

Understanding Template Literal Type Inference

Given this function, what is the inferred return type of makeKey('user', 42)?

Typescript

function makeKey<T extends string | number, U extends string | number>(prefix: T, id: U) {
  return `${prefix}_${id}` as const;
}

const key = makeKey('user', 42);

// What is the type of key?

A `${string}_${number}`

B string

C "user_42"

D `${T}_${U}`

💡 Hint

The function uses as const to fix the return type.

The as const assertion tells TypeScript to infer the exact string literal type of the returned template literal, so key has the type "user_42".

❓ Predict Output

expert

Complex Template Literal Type with Conditional Types

What is the type of Result after this code?

Typescript

type Status = "success" | "error";

type Response<T extends Status> = T extends "success"
  ? `Response: ${T} with data`
  : `Response: ${T} with error`;

// What is Response<"error">?

type Result = Response<"error">;

A "Response: error with error"

B "Response: error with data"

C "Response: success with error"

D "Response: success with data"

💡 Hint

Conditional types choose the template based on the input type.

For T = "error", the conditional type picks the false branch, so Result is the string literal type "Response: error with error".

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