Phantom types in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a phantom type in TypeScript?

A phantom type is a type parameter used only at compile time to add extra type safety without affecting the runtime code. It helps catch errors early by distinguishing types that are otherwise similar.

beginner

Why use phantom types instead of regular types?

Phantom types let you add extra compile-time checks without adding runtime overhead. They help prevent mixing incompatible types that look similar but represent different concepts.

intermediate

How do phantom types affect the generated JavaScript code?

Phantom types do not appear in the generated JavaScript code because they are only used by the TypeScript compiler for type checking. They have zero runtime cost.

intermediate

Example: What does this TypeScript code do?
<pre>type Meter = { value: number; _unit: 'meter' };
type Second = { value: number; _unit: 'second' };
function addMeters(a: Meter, b: Meter): Meter {
  return { value: a.value + b.value, _unit: 'meter' };
}</pre>

This code uses phantom types (_unit) to distinguish Meter and Second types. The addMeters function only accepts Meter types, preventing accidental mixing with Second types at compile time.

intermediate

Can phantom types be used with generics in TypeScript?

Yes, phantom types are often used as generic type parameters that do not appear in runtime values but help enforce constraints and improve type safety during compilation.

What is the main purpose of phantom types in TypeScript?

A Add extra runtime checks

B Add extra compile-time type safety without runtime cost

C Improve performance of JavaScript code

D Generate additional JavaScript code

Phantom types add compile-time safety but do not affect runtime code.

Which of these is true about phantom types?

A They are erased during compilation

B They are used to store runtime data

C They increase the size of the compiled JavaScript

D They exist only at runtime

Phantom types are erased during compilation and do not exist at runtime.

How can phantom types help prevent bugs?

A By running extra tests automatically

B By optimizing memory usage

C By enforcing different types for similar data at compile time

D By logging errors at runtime

Phantom types distinguish similar data types to avoid mixing them accidentally.

In this code snippet, what is '_unit' used for?

type Meter = { value: number; _unit: 'meter' };

A To store the unit at runtime

B To calculate values

C To convert units

D As a phantom type to distinguish Meter from other types

The '_unit' property is a phantom type marker used only for compile-time checks.

Can phantom types be used with generic functions?

A Yes, to add compile-time constraints without runtime impact

B No, they only work with concrete types

C Only if the generic is a number

D Only if the generic is a string

Phantom types are often used as generic parameters to enforce constraints at compile time.

Explain what phantom types are and why they are useful in TypeScript.

Think about types that exist only during checking, not when the program runs.

Describe a simple example where phantom types prevent mixing incompatible units like meters and seconds.

Use a property that marks the unit but is not used at runtime.