Patterns

How do I make a helper infer a literal tuple without requiring callers to write `as const`?

Mark the type parameter `const`; inference then prefers readonly literal shapes over widened arrays and primitives.

The recipe

function tuple<const T extends readonly unknown[]>(...values: T): T {
  return values
}

const statuses = tuple('idle', 'loading', 'done')
const point = tuple(10, 20)

The build compiles this and checks each result below.

How it works

01
```
const T extends readonly unknown[]
```
`const` changes the inference preference; the readonly constraint supplies a compatible narrow target.
02
```
...values: T
```
The rest parameter records each argument as one tuple position.

What you get

```
typeof statuses
```
→readonly ["idle", "loading", "done"]
```
(typeof statuses)[number]
```
→"idle" | "loading" | "done"
```
typeof point
```
→readonly [10, 20]

Where it goes wrong

Const inference is shallow and preference-based. Values already stored in widened variables stay widened, and mutable constraints can force the compiler back to mutable arrays.

Takeaway

Use const type parameters at API boundaries where callers benefit from literal precision without extra syntax.