How do you specify that a class property is an integer?

Question

I\'m experimenting with TypeScript, and in the process of creating a class with an ID field that should be an integer, I have gotten a

Answer 1

This is the top result on Google for me so I figure I should provide the solutions I found.

Using bigint

Now that it's 2020 and bigint has been accepted, it deserves a mention. You can simply do the below. Beware that bigints come with a bigger performance impact compared to a number.

const myNumber: bigint = 10n

Using a nominal type / tagged type / opaque type

An alternative is to use a nominal type, but it's arguably less ergonomic and I'm not sure if it's any faster than bigint, but the pattern does generalise to any type, not just number. TypeScript doesn't have "first-class" support for this so you have to do a cheeky hack. There's a library for this called newtype-ts that includes common types like Integer so you might just want to just use that, but I'll explain the workings below.

To start out with we define the integer type.

const TAG = Symbol()
type integer = number & { readonly [TAG]: unique symbol }

The TAG ensures we have a unique value so that we don't accidentally make an object with the same key, and we make the field a unique symbol too for the same reason. Now, your integer won't actually have this object field but that's fine.

With this you can still add integer to number using +. Not good. So you can enforce type safety on the arguments here by massaging the type system with a function. I'm just gonna call it guard, and again as you can see it isn't specific to integers – you could make more opaque types and use this again.

type guard = (f: (...ns: Array) => A, ...ns: Array) => A
const guard: guard = (f, ...ns) => f(...ns)

If you try to call that with a number

const bad: integer = guard((a, b) => a + b as integer, myCoolInteger, 10)

you'll get an error like below

Argument of type '10' is not assignable to parameter of type 'integer'.
  Type '10' is not assignable to type '{ readonly [TAG]: unique symbol; }'.(2345)

Note that you aren't enforcing the return type here (because you have to use as integer) and some operators like / will return floating point numbers so you probably still want to do runtime checks or add a Math.round to a specialised version of guard, but this will at least ensure you're not trying to use two separate numeric types together – imagine you have GBP and USD and try to add those, that would likely not be what you intended.