A Beginner's Guide to Generics in Typescript

Can’t quite wrap your head around generics in typescript? The best way to explain generics, in my opinion, is by analogy.

Consider a courier service like UPS or Puralator that delivers packages around the world. The courier service doesn’t need to know the exact nature of the parcel - they don’t need to know if it’s a toy, book, clothing item, etc., - but the expectation is that the recipient will receive that which was sent. If a toy is sent, a toy will be received. The courier is in charge of delivering that package, but doesn’t need to know the exact type of content that it’s delivering.

In this analogy, the courier service is our function and the package is our generic. Let’s try to clarify this analogy with some code. This example (which doesn’t use generics) shows a courier function which receives a string as an argument, and returns a string. Our return value delivery is correctly typed as a string.

function courier(package: string): string {
  return package
}

// delivery is typed as a string
const delivery = courier('book')

But presumably we want our courier function to be able to handle more than just strings. We should be able to handle numbers, objects, or anything…

We might be tempted to achieve this using the any type.

function courier(package: any): any {
  return package
}

// delivery is typed as any
const delivery = courier('book')

This works, but now our delivery value is typed as any. But shouldn’t the caller of the function - the person sending the package - know the type of the package returned? In other words, wouldn’t it be nice if our delivery value retained whatever type the package was initially assigned? If our package is a string, our delivery should be a string. If it’s a number, our delivery should be a number… and so on.

This is where generics come in. Let’s write the same function using generics:

function courier<T>(package: T): T {
  return package
}

// delivery is typed as string
const delivery = courier('book')

// lucknumber is typed as number
const luckynumber = courier(7)

T is a type variable. You don’t need to use T, but it’s standard practice. Basically, this syntax reads as: “Courier accepts a generic type, represented as T. This generic type is defined by our package argument, and our courier function returns a value of the same type.”

Maybe our courier should be able to accept multiple packages at once.

function courier<T>(package: T[]): T[] {
  return package
}

// typed as [string, number]
const delivery = courier(['book', 7])

And there you have it. Our courier function now accepts an arbitrarily long array of packages, and correctly types that array in the return value.

This is just the start, but hopefully you see how generics are particularly useful for creating reusable, flexible functions and classes.

(This is an article posted to my blog at benmcmahen.com. You can read it online by clicking here.)