# TypeScript Positive Integer Type

This is how you define the `PositiveInteger` dependent type in TypeScript without using type predicate functions or branded (tagged) types:

```typescript
type PositiveInteger<T extends number> =
  `${T}` extends '0' | `-${any}` | `${any}.${any}` ? never : T
```

It can then be used as follows:

```typescript
function test<T extends number>(n: PositiveInteger<T>) { /***/ }
```

The function's argument will be correctly checked at compile time, as shown here:

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1675201754238/7d7f19a3-7a38-4e6b-8ba3-47068fa99c55.png?width=399 align="left")

Things that can't be checked at compile time, such as variables, are passed through. This is obviously unsolvable without resorting to runtime checks (type predicates), because TypeScript types don't exist at runtime.

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1675202197633/82e00a8a-2337-4231-8fed-e3e77df0bfba.png?width=516 align="left")

Similarly, a non-negative integer type, aka `unsigned int`, can be defined as follows:

```typescript
type NonnegativeInteger<T extends number> =
  `${T}` extends `-${any}` | `${any}.${any}` ? never : T
```

Both the `PositiveInteger` and `NonnegativeInteger` types will be available in the next version (0.1.10) of [natlib](https://github.com/mvasilkov/natlib). They can be found in the `prelude` module.

```typescript
// npm i natlib
import type { PositiveInteger, NonnegativeInteger } from './node_modules/natlib/prelude'
```

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Implementation note: all instances of `any` in the template literal types can be changed to `string` or even `infer _` — this shouldn't matter.