Template literal with unions in Typescript - Mini Project: Build & Apply

Template literal with unions

📖 Scenario: You are building a simple greeting system that creates personalized messages based on a person's name and the time of day.

🎯 Goal: Create a TypeScript program that uses template literal types combined with union types to generate greeting messages.

📋 What You'll Learn

Use union types for names and times of day

Use a template literal type to combine the unions into greeting messages

Create a variable with the combined greeting type

Print the greeting message

💡 Why This Matters

🌍 Real World

Template literal types with unions help create precise string types for things like personalized messages, URLs, or CSS class names.

💼 Career

Understanding template literal types is useful for TypeScript developers to write safer and more expressive code, especially in UI and API development.

1

Create union types for names and times of day

Create a union type called Name with the values 'Alice' and 'Bob'. Then create a union type called TimeOfDay with the values 'morning' and 'evening'.

Typescript

# Create union types Name and TimeOfDay
// Your code here

Hint

Use the pipe symbol | to create union types in TypeScript.

2

Create a template literal type for greetings

Create a template literal type called Greeting that combines Name and TimeOfDay into strings like 'Good morning, Alice!' or 'Good evening, Bob!'.

Typescript

type Name = 'Alice' | 'Bob';
type TimeOfDay = 'morning' | 'evening';
// Create template literal type Greeting
// Your code here

Hint

Use backticks ` and ${} to create template literal types.

3

Create a variable with a greeting message

Create a variable called message of type Greeting and assign it the value 'Good morning, Alice!'.

Typescript

type Name = 'Alice' | 'Bob';
type TimeOfDay = 'morning' | 'evening';
type Greeting = `Good ${TimeOfDay}, ${Name}!`;
// Create variable message with type Greeting and value 'Good morning, Alice!'
// Your code here

Hint

Declare the variable with const and specify the type Greeting.

4

Print the greeting message

Use console.log to print the variable message.

Typescript

type Name = 'Alice' | 'Bob';
type TimeOfDay = 'morning' | 'evening';
type Greeting = `Good ${TimeOfDay}, ${Name}!`;

const message: Greeting = 'Good morning, Alice!';
// Print the message
// Your code here

Hint

Use console.log(message); to print the message to the console.

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