Patterns

How do I index a discriminated union by its `kind` instead of repeatedly extracting members?

Map over the union and remap each member to the literal value of its discriminant.

The recipe

type ByKind<Union extends { kind: PropertyKey }> = {
  [Member in Union as Member['kind']]: Member
}

type Action =
  | { kind: 'add'; amount: number }
  | { kind: 'reset' }
  | { kind: 'rename'; name: string }

type ActionMap = ByKind<Action>

The build compiles this and checks each result below.

How it works

01
```
[Member in Union
```
Mapped types can iterate a union of objects, not only a union of property keys.
02
```
as Member['kind']
```
Key remapping promotes each discriminant value to a property name.

What you get

```
ActionMap['add']
```
→{ kind: "add"; amount: number; }
```
ActionMap['reset']
```
→{ kind: "reset"; }
```
keyof ActionMap
```
→"add" | "reset" | "rename"

Where it goes wrong

Every discriminant must be a unique `PropertyKey`. If two members share the same `kind`, their values merge under one property and the lookup no longer identifies a single member.

Takeaway

Key remapping turns a union’s discriminant into an index for constant-time type lookup.