Branding & Flavoring

Introduction

TypeScript language mainly uses structural typing. In structural typing, an element is considered to be compatible with another if, for each feature within the second element's type, a corresponding and identical feature exists in the first element's type. Sometimes this behavior may allow us to write erroneous code.

Example

Let's imagine a function which calculates candy price based on their weight and price per unit:

// Create a type for our candy
type Candy = {
  pricePerUnit: number
  weight: number
}

// Create a function from our example
const calculateCandyPrice = (candy: Candy) => candy.pricePerUnit * candy.weight

// Create a type with similar structure to type Candy
type Veggie = {
  pricePerUnit: number
  weight: number
}

// Declare variables based on our types
declare const candyDrops: Candy
declare const broccolis: Veggie

// Is this function safely typed? 
calculateCandyPrice(candyDrops)                          // OK 
calculateCandyPrice({ pricePerUnit: 1000, weight: 20 })  // OK 
calculateCandyPrice(broccolis)                           // OK 

TypeScript compiler does not throw an error, even though we as programmes see that this behavior may not be desired. This problem can be solved by nominal typing. Nominal typing means that two variables are type-compatible if and only if their declarations name the same type. TypeScript unfortunately does not have such a syntax feature. There are some methods which we can use to omit that problem.

Branding

Branding concept is a technique in which we add a unique field which will make our type differ from another types. What is important we do not need to assign any value there because it only exists for TypeScript compiler.

type Candy = {
  __brand: "Candy"
  pricePerUnit: number
  weight: number
}

type Veggie = {
  __brand: "Veggie"
  pricePerUnit: number
  weight: number
}

const calculateCandyPrice = (candy: Candy) => candy.pricePerUnit * candy.weight

declare const candyDrops: Candy
declare const broccolis: Veggie

calculateCandyPrice(candyDrops)                          // OK 
calculateCandyPrice({ pricePerUnit: 1000, weight: 20 })  // Error
calculateCandyPrice(broccolis)                           // Error

We can also create generic type for our branding:

type Brand<T, BrandT> = T & { __brand: BrandT }

// Similar type as before
type Candy = Brand<{ pricePerUnit: number, weight: number }, "Candy">

Changing primitive value to branded value requires manual casting:

type CandyID = Brand<string, "CandyID">

const candyId = "some-long-id" as CandyID // Brand<string, "CandyID">

To make our code cleaner we can use these helpers to change our type to be nominal or reversed:

const toCandyID = (id: string) => id as CandyID
const fromCandyID = (id: CandyID) => id as string

const candyIdTo = toCandyID('some-long-id-1') // Brand<string, "CandyID">
const candyIdFrom = fromCandyID(candyIdTo)    // string

Bear in mind that branding has got two cons:

  1. Compiler allow us to read __brand property
const brandProperty = candyDrops.__brand // OK 
  1. A raw object passed to the function as an argument stopped working
calculateCandyPrice({ pricePerUnit: 1000, weight: 20 })  // Error

Flavoring

Flavoring concept is a technique in which we add a unique optional field which will make our type differ from another types. This difference between branding and flavoring allow us to pass a raw object as an argument to the function. Unfortunately we still can access __flavor property.

type Candy = {
  __flavor?: "Candy"
  pricePerUnit: number
  weight: number
}

type Veggie = {
  __flavor?: "Veggie"
  pricePerUnit: number
  weight: number
}

const calculateCandyPrice = (candy: Candy) => candy.pricePerUnit * candy.weight

declare const candyDrops: Candy
declare const broccolis: Veggie

calculateCandyPrice(candyDrops)                          // OK 
calculateCandyPrice({ pricePerUnit: 1000, weight: 20 })  // OK
calculateCandyPrice(broccolis)                           // Error

We can also create generic type for our flavoring:

type Flavor<T, FlavorT> = T & { __flavor?: FlavorT }

// Similar type as before
type Veggie = Flavor<{ pricePerUnit: number, weight: number }, "Veggie">

We do not need to cast primitive values manually using this technique.

Conclusion

To sum up those two techniques - it is commonly used that if we use branding and flavoring we use branding for primitive types while flavoring for objects. We can use conditional type for that:

type Nominal<T, NameT> = T extends object ? Flavor<T, NameT> : Brand<T, NameT>

Alternatively we can use other more sophisticated techniques to achieve nominal typing or libraries such as: newtype-ts or io-ts.

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