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Document type helpers for oneOfs #1023

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Mar 29, 2024
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Document type helpers for oneOfs #1023

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22 changes: 22 additions & 0 deletions README.markdown
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Expand Up @@ -724,6 +724,28 @@ As this will automatically enforce only one of `field_a` or `field_b` "being set

In ts-proto's currently-unscheduled 2.x release, `oneof=unions` will become the default behavior.

## OneOf Type Helpers

The following helper types may make it easier to work with the types generated from `oneof=unions`:

```ts
/** Extracts all the case names from a oneOf field. */
type OneOfCases<T> = T extends { $case: infer U extends string } ? U : never;

/** Extracts a union of all the value types from a oneOf field */
type OneOfValues<T> = T extends { $case: infer U extends string; [key: string]: unknown }
? T[U]
: never;

/** Extracts the specific type of a oneOf case based on its field name */
type OneOfCase<T, K extends OneOfCases<T>> = T extends {
$case: infer U extends K;
[key: string]: unknown;
}
? T[U]
: never;
```

# Default values and unset fields

In core Protobuf (and so also `ts-proto`), values that are _unset_ or equal to the default value are not sent over the wire.
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