Template literal with unions in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a type that combines 'hello' and 'world' with a space.

Typescript

type Greeting = `$[1] ${'world'}`;

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

A'hello'

B'hi'

C'hey'

D'hola'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name instead of a string literal.

Forgetting quotes around the string literal.

The template literal type combines the string literal 'hello' with 'world' separated by a space.

2fill in blank

medium

Complete the code to define a type that can be 'red', 'green', or 'blue' using a union.

Typescript

type Color = [1];

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

A'red', 'green', 'blue'

B'red' & 'green' & 'blue'

C'red' | 'green' | 'blue'

D['red', 'green', 'blue']

Attempts:

3 left

💡 Hint

Common Mistakes

Using commas instead of pipes.

Using ampersands which mean intersection, not union.

The union type uses the pipe symbol '|' to combine string literals into one type.

3fill in blank

hard

Fix the error in the template literal type that combines 'cat' or 'dog' with 'food' or 'toy'.

Typescript

type PetItem = `$[1] ${'food' | 'toy'}`;

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

A'cat' & 'dog'

B['cat', 'dog']

C'cat', 'dog'

D'cat' | 'dog'

Attempts:

3 left

💡 Hint

Common Mistakes

Using '&' which means intersection, not union.

Using commas or array syntax which are invalid here.

The union operator '|' correctly combines 'cat' and 'dog' as possible string literals.

4fill in blank

hard

Fill both blanks to create a type for 'small', 'medium', or 'large' sizes with 'shirt' or 'pants'.

Typescript

type Clothing = `$[1]`$[2];

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

A'small' | 'medium' | 'large'

B'shirt' | 'pants'

C ' '

D'-'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a dash '-' instead of a space.

Putting the clothing union in the first blank.

The first blank is the size union, the second blank is a space string to separate size and clothing type.

5fill in blank

hard

Fill all three blanks to create a type for 'red', 'green', or 'blue' colors combined with 'circle' or 'square' shapes, separated by a dash.

Typescript

type ShapeColor = `$[1]$[2]$[3]`;

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

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

B'-'

C'circle' | 'square'

D'_'

Attempts:

3 left

💡 Hint

Common Mistakes

Using underscore '_' instead of dash '-' as separator.

Mixing up the order of colors and shapes.

The first blank is the color union, the second blank is the dash separator, and the third blank is the shape union.

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