Patterns

How do I get only the keys whose properties are strings, functions, or another chosen type?

Map every key to itself or `never`, then index the mapped object to collect the survivors.

The recipe

type KeysByValue<T, Value> = {
  [K in keyof T]-?: T[K] extends Value ? K : never
}[keyof T]

type Model = {
  id: number
  name: string
  email: string
  active: boolean
}

type TextKeys = KeysByValue<Model, string>

The build compiles this and checks each result below.

How it works

01
```
T[K] extends Value ? K : never
```
Matching properties keep their key; non-matches disappear into `never`.
02
```
}[keyof T]
```
Indexing turns the property results into one union of keys.

What you get

```
TextKeys
```
→"name" | "email"
```
KeysByValue<Model, number>
```
→"id"
```
KeysByValue<Model, Date>
```
→never

Where it goes wrong

The check is assignability, not exact equality. A literal string property also extends `string`, and a union property matches only when the whole union extends the requested value type.

Takeaway

Mapped filtering is map to key-or-never, then index to collapse the result.