Template literal with unions in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a template literal type in TypeScript?

A template literal type lets you create new string types by combining unions of string literals with template strings, similar to how template literals work in JavaScript but at the type level.

intermediate

How do unions work with template literal types?

Unions allow multiple string literal types to be combined. When used inside a template literal type, TypeScript creates all possible combinations of those union members as new string literal types.

beginner

Example: What is the resulting type of `type Greeting = `${'Hello' | 'Hi'}, ${'Alice' | 'Bob'}``?

The type Greeting is a union of these string literals: 'Hello, Alice', 'Hello, Bob', 'Hi, Alice', and 'Hi, Bob'.

intermediate

Why use template literal types with unions?

They help create precise string types that represent all valid combinations, improving type safety and autocomplete suggestions in your code.

advanced

Can template literal types with unions be nested?

Yes, you can nest unions inside template literals multiple times to build complex string types representing many combinations.

What does the TypeScript type `type T = `${'a' | 'b'}${'1' | '2'}`` represent?

A 'a1' & 'a2' & 'b1' & 'b2'

B 'a' | 'b' | '1' | '2'

C A single string 'ab12'

D 'a1' | 'a2' | 'b1' | 'b2'

The template literal with unions creates all combinations of the union members, so it results in the union of 'a1', 'a2', 'b1', and 'b2'.

Which keyword is used to combine multiple string literal types in TypeScript?

A union (|)

B intersection (&)

C extends

D implements

The union operator (|) combines multiple string literal types into one union type.

Can template literal types only use string literals inside the template?

A Only numbers can be used

B No, any type can be used

C Yes, only string literals or unions of string literals

D Only boolean values can be used

Template literal types work with string literals or unions of string literals to produce new string literal types.

What is the benefit of using template literal types with unions?

A They create precise string types representing all valid combinations

B They improve runtime performance

C They allow dynamic string concatenation at runtime

D They replace interfaces

Template literal types with unions help define exact string types for better type safety and autocomplete.

If you have `type Colors = 'red' | 'blue'` and `type Shades = 'light' | 'dark'`, what does `type ColoredShade = `${Shades} ${Colors}`` produce?

A 'red light' | 'blue light' | 'red dark' | 'blue dark'

B 'light red' | 'light blue' | 'dark red' | 'dark blue'

C 'lightred' | 'lightblue' | 'darkred' | 'darkblue'

D An error

The template literal combines each shade with each color separated by a space, producing all combinations.

Explain how template literal types work with unions in TypeScript and give a simple example.

Think about how strings combine in JavaScript template literals but at the type level.

Describe a practical use case where template literal types with unions improve your TypeScript code.

Consider situations where you want to restrict strings to specific patterns.

Practice

1.

Which of the following best describes a template literal with unions in TypeScript?

easy

A. A method to concatenate numbers and strings without type checking.

B. A way to create specific string patterns by combining fixed text and union types.

C. A function that returns a template string without any type restrictions.

D. A way to declare variables without specifying their types.

Solution

1. Step 1: Understand template literals and unions

   Template literals allow embedding expressions in strings. Union types restrict values to a set of options.

2. Step 2: Combine both concepts

   Template literal with unions creates strings with fixed parts and limited variable parts, improving type safety.

3. Final Answer:

   A way to create specific string patterns by combining fixed text and union types. -> Option B

4. Quick Check:

   Template literal + union = specific string patterns [OK]

Hint: Think of fixed text plus limited choices in strings [OK]

Common Mistakes:

• Confusing unions with simple string concatenation
• Ignoring type safety benefits
• Thinking it's only about functions

2.

Which of the following TypeScript declarations correctly uses a template literal with unions?

type Size = "small" | "medium" | "large";
type Message = `Size is ${Size}`;

easy

A. type Message = `Size is ${string}`;

B. type Message = `Size is ${boolean}`;

C. type Message = `Size is ${number}`;

D. type Message = `Size is ${Size}`;

Solution

1. Step 1: Check the union type used inside template literal

   The union type Size is "small" | "medium" | "large" and should be used inside the template literal.

2. Step 2: Match the template literal syntax

   The correct syntax is `Size is ${Size}` to restrict the string to those sizes.

3. Final Answer:

   type Message = `Size is ${Size}`; -> Option D

4. Quick Check:

   Use union type inside template literal exactly [OK]

Hint: Use the union type name inside ${} exactly [OK]

Common Mistakes:

• Replacing union with general types like string or number
• Using incorrect syntax for template literals
• Forgetting backticks around the template

3.

What is the type of result in the following code?

type Color = "red" | "green" | "blue";
type Alert = `Alert: ${Color}`;

const result: Alert = "Alert: green";

medium

A. "Alert: red" | "Alert: green" | "Alert: blue"

B. "Alert: red" | "Alert: green"

C. "Alert: green"

D. string

Solution

1. Step 1: Understand the template literal with union

   The type Alert creates strings by combining "Alert: " with each value in Color.

2. Step 2: Expand the union to all possible strings

   So Alert is the union of "Alert: red", "Alert: green", and "Alert: blue".

3. Final Answer:

   "Alert: red" | "Alert: green" | "Alert: blue" -> Option A

4. Quick Check:

   Template literal expands union to all combined strings [OK]

Hint: Expand union inside template literal to all string combos [OK]

Common Mistakes:

• Assuming only one string value is allowed
• Confusing union with single string
• Using general string type instead of union

4.

Identify the error in this TypeScript code using template literals with unions:

type Status = "success" | "error";
type Message = `Status: ${string}`;

const msg: Message = "Status: success";

medium

A. Using string inside template literal instead of union type Status.

B. Missing backticks around the string value assigned to msg.

C. Incorrect union type declaration for Status.

D. Assigning a string literal to a non-string type.

Solution

1. Step 1: Check the template literal type definition

   The template literal uses ${string} which allows any string, not restricted to Status.

2. Step 2: Identify the correct usage

   It should use ${Status} to restrict values to "success" or "error" only.

3. Final Answer:

   Using string inside template literal instead of union type Status. -> Option A

4. Quick Check:

   Use union type inside template literal for safety [OK]

Hint: Use union type inside ${}, not general string [OK]

Common Mistakes:

• Using string instead of union type inside template
• Ignoring type safety benefits
• Confusing string literals with template literals

5.

Given the following types, which of these assignments is valid without TypeScript errors?

type Direction = "up" | "down";
type Action = "move" | "stop";
type Command = `${Action}-${Direction}`;

const cmd1: Command = "move-up";
const cmd2: Command = "stop-left";
const cmd3: Command = "move-down";
const cmd4: Command = "jump-up";

hard

A. Only cmd2 is valid.

B. All commands are valid.

C. Only cmd1 and cmd3 are valid.

D. Only cmd4 is valid.

Solution

1. Step 1: Understand the Command type

   Command is a template literal combining Action and Direction with a dash, so valid strings are "move-up", "move-down", "stop-up", "stop-down".

2. Step 2: Check each assignment

   cmd1 = "move-up" (valid), cmd2 = "stop-left" (invalid, "left" not in Direction), cmd3 = "move-down" (valid), cmd4 = "jump-up" (invalid, "jump" not in Action).

3. Final Answer:

   Only cmd1 and cmd3 are valid. -> Option C

4. Quick Check:

   Match both parts to union values exactly [OK]

Hint: Check both parts of template literal match union values [OK]

Common Mistakes:

• Assuming any string with dash is valid
• Ignoring union type restrictions
• Confusing Action and Direction values