Patterns

How do I select one property from every object and have the returned array keep that property’s type?

Bind the key to `keyof T` and use indexed access for the return element.

The recipe

function pluck<T, K extends keyof T>(
  items: readonly T[],
  key: K,
): T[K][] {
  return items.map(item => item[key])
}

const people = [
  { id: 1, name: 'Ada' },
  { id: 2, name: 'Lin' },
]

const names = pluck(people, 'name')
const ids = pluck(people, 'id')

The build compiles this and checks each result below.

How it works

01
```
K extends keyof T
```
The selected property must exist on every element described by `T`.
02
```
T[K][]
```
The selected key determines the element type of the returned array.

What you get

```
typeof names
```
→string[]
```
typeof ids
```
→number[]
```
(typeof names)[number]
```
→string

Where it goes wrong

For a union of differently shaped objects, `keyof T` contains only keys safe on every member. Narrow the union first or model a discriminated operation when branch-specific keys are needed.

Takeaway

Correlate a key parameter with `T[K]` whenever a property choice determines a result type.