Template literal with unions in Typescript

Introduction

Template literals with unions let you create flexible string types by combining fixed parts with a set of options. This helps catch mistakes and makes your code clearer.

When you want to allow only specific string patterns made from fixed words and choices.

When you want to build URLs or commands that follow certain formats.

When you want to restrict input to a few allowed string combinations.

When you want to improve code readability by naming common string patterns.

When you want TypeScript to check your strings for correctness automatically.

Syntax

Typescript

type Greeting = `Hello, ${'Alice' | 'Bob' | 'Eve'}`;

The backticks `` create a template literal type.

The ${} part lets you insert a union of string options.

Examples

This creates strings like 'Size is small', 'Size is medium', or 'Size is large'.

Typescript

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

This creates strings like 'red-on', 'green-off', etc.

Typescript

type Color = 'red' | 'green';
type Status = 'on' | 'off';
type Light = `${Color}-${Status}`;

This creates strings like 'move-up' or 'move-down'.

Typescript

type Direction = 'up' | 'down';
type Command = `move-${Direction}`;

Sample Program

This program defines an order type combining quantity and fruit. It prints the order if it matches allowed patterns. Trying to use a wrong quantity causes an error.

Typescript

type Fruit = 'apple' | 'banana' | 'cherry';
type Quantity = 'one' | 'two' | 'three';
type Order = `${Quantity} ${Fruit}`;

function printOrder(order: Order) {
  console.log(`You ordered: ${order}`);
}

printOrder('two banana');
// printOrder('four apple'); // Error: 'four' not allowed

Important Notes

Template literal types only work with string unions inside ${}.

TypeScript checks these types at compile time, not at runtime.

You can combine multiple unions to create many string options easily.

Summary

Template literal with unions helps create specific string patterns.

It improves code safety by limiting allowed string values.

Use it to combine fixed text with choices for clear, checked strings.

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