Phantom types in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a phantom type parameter in a TypeScript interface.

Typescript

interface Box<[1]> {
  value: string;
}

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

AT

Bstring

Cnumber

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using a concrete type like 'number' instead of a generic type parameter.

Forgetting to declare a generic parameter.

The phantom type parameter is a generic type parameter that is not used in the interface's members. Here, T is a placeholder type parameter.

2fill in blank

medium

Complete the code to define a phantom type in a TypeScript type alias.

Typescript

type TaggedString<[1]> = string & { __tag?: [1] };

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

Anumber

Bboolean

Cstring

DTag

Attempts:

3 left

💡 Hint

Common Mistakes

Using a concrete type like 'string' instead of a generic parameter.

Not using a generic parameter at all.

The phantom type parameter Tag is used to create a unique branded type without affecting the runtime value.

3fill in blank

hard

Fix the error in the phantom type usage by completing the generic parameter.

Typescript

function wrapValue<[1]>(value: string): TaggedString<[1]> {
  return value as TaggedString<[1]>;
}

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

Anumber

Bstring

CT

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using a concrete type like 'string' instead of a generic parameter.

Omitting the generic parameter declaration.

The generic parameter T must be declared to use the phantom type correctly in the function.

4fill in blank

hard

Fill both blanks to create a phantom type that distinguishes between UserId and OrderId.

Typescript

type [1] = TaggedString<'UserId'>;
type [2] = TaggedString<'OrderId'>;

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

AUserId

Bstring

COrderId

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the type names and tags.

Using concrete types like 'string' or 'number' instead of phantom types.

Phantom types UserId and OrderId are created to distinguish between different string types at compile time.

5fill in blank

hard

Fill all three blanks to define a function that accepts only UserId and returns a string.

Typescript

function getUserName(id: [1]): [2] {
  // pretend to fetch user name
  return 'User_' + id as [3];
}

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

AOrderId

Bstring

CUserId

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using OrderId instead of UserId as parameter type.

Returning a phantom type instead of a string.

The function parameter must be UserId phantom type, and the return type is string. The cast uses string as well.

Practice

1. What is the main purpose of using phantom types in TypeScript?

easy

A. To improve performance by removing types

B. To create new objects at runtime

C. To add extra type information without affecting runtime data

D. To allow dynamic typing in TypeScript

Solution

1. Step 1: Understand phantom types concept

   Phantom types add extra type information that exists only at compile time and does not affect the actual data or runtime behavior.

2. Step 2: Compare options with concept

   Only To add extra type information without affecting runtime data correctly states that phantom types add extra type info without changing runtime data. Other options describe unrelated concepts.

3. Final Answer:

   To add extra type information without affecting runtime data -> Option C

4. Quick Check:

   Phantom types = extra compile-time info [OK]

Hint: Phantom types add info only at compile time, no runtime change [OK]

Common Mistakes:

• Thinking phantom types create new objects
• Confusing phantom types with runtime data changes
• Believing phantom types improve runtime performance

2. Which of the following is the correct way to declare a phantom type in TypeScript?

easy

A. type UserId = string & { readonly __brand: unique symbol }

B. type UserId = string | number

C. type UserId = string[]

D. type UserId = number

Solution

1. Step 1: Identify phantom type syntax

   Phantom types use intersection with a unique symbol or branded property to add phantom info without changing the base type.

2. Step 2: Check each option

   type UserId = string & { readonly __brand: unique symbol } uses intersection with a unique symbol brand, which is the correct phantom type pattern. Others are normal types or unions.

3. Final Answer:

   type UserId = string & { readonly __brand: unique symbol } -> Option A

4. Quick Check:

   Phantom type = base type & unique brand [OK]

Hint: Phantom types use intersection with unique symbol branding [OK]

Common Mistakes:

• Using union types instead of intersection for phantom types
• Declaring phantom types as arrays or simple types
• Missing the unique symbol branding

3. What will be the result of this TypeScript code?

type Meter = number & { readonly __unit: 'meter' };
function addDistance(a: Meter, b: Meter): Meter {
  return (a + b) as Meter;
}
const dist1 = 5 as Meter;
const dist2 = 10 as Meter;
const total = addDistance(dist1, dist2);
console.log(total);

medium

A. NaN

B. 15

C. Type error at addDistance call

D. undefined

Solution

1. Step 1: Understand phantom type behavior at runtime

   Phantom types exist only at compile time. At runtime, they are just numbers. So adding two numbers works normally.

2. Step 2: Analyze code output

   dist1 and dist2 are numbers with phantom type. Adding them returns 15, which is logged.

3. Final Answer:

   15 -> Option B

4. Quick Check:

   Phantom types don't affect runtime value [OK]

Hint: Phantom types vanish at runtime; values behave normally [OK]

Common Mistakes:

• Expecting type errors at runtime
• Thinking phantom types change runtime values
• Confusing phantom types with runtime wrappers

4. Identify the error in this phantom type usage:

type UserId = string & { readonly __brand: unique symbol };
function getUser(id: UserId) {
  return `User: ${id}`;
}
const id = 'abc123';
getUser(id);

medium

A. Function getUser should return a number, not string

B. No error, code runs fine

C. Unique symbol cannot be used in type intersections

D. Passing a plain string instead of branded UserId causes a type error

Solution

1. Step 1: Check argument type for getUser

   getUser expects a UserId (string with phantom brand). Passing plain string 'abc123' lacks the brand, causing a type error.

2. Step 2: Validate other options

   Return type is correct, unique symbol usage is valid, and code does not run fine due to type mismatch.

3. Final Answer:

   Passing a plain string instead of branded UserId causes a type error -> Option D

4. Quick Check:

   Phantom type mismatch = type error [OK]

Hint: Phantom types require branded values, not plain base types [OK]

Common Mistakes:

• Ignoring phantom brand when passing values
• Thinking unique symbol is invalid in types
• Assuming no error when types differ

5. How can phantom types help prevent mixing different ID types in a TypeScript project?
Consider these types:

type UserId = string & { readonly __brand: 'UserId' };
type ProductId = string & { readonly __brand: 'ProductId' };

What is the main benefit of this approach?

hard

A. It prevents accidentally using a ProductId where a UserId is expected at compile time

B. It improves runtime performance by optimizing string storage

C. It automatically converts between UserId and ProductId

D. It allows mixing IDs freely without errors

Solution

1. Step 1: Understand phantom branding for IDs

   Phantom types add unique brands to similar base types (string) to distinguish them at compile time.

2. Step 2: Identify benefit in type safety

   This branding prevents passing a ProductId where a UserId is expected, catching errors before running code.

3. Final Answer:

   It prevents accidentally using a ProductId where a UserId is expected at compile time -> Option A

4. Quick Check:

   Phantom types enforce distinct types safely [OK]

Hint: Phantom types catch ID mix-ups at compile time [OK]

Common Mistakes:

• Thinking phantom types affect runtime performance
• Assuming automatic conversion between branded types
• Believing phantom types allow mixing without errors