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Contents
import type {IfNever} from './if-never';
/**
Extract the keys from a type where the value type of the key extends the given `Condition`.
Internally this is used for the `ConditionalPick` and `ConditionalExcept` types.
@example
```
import type {ConditionalKeys} from 'type-fest';
interface Example {
a: string;
b: string | number;
c?: string;
d: {};
}
type StringKeysOnly = ConditionalKeys<Example, string>;
//=> 'a'
```
To support partial types, make sure your `Condition` is a union of undefined (for example, `string | undefined`) as demonstrated below.
@example
```
import type {ConditionalKeys} from 'type-fest';
type StringKeysAndUndefined = ConditionalKeys<Example, string | undefined>;
//=> 'a' | 'c'
```
@category Object
*/
export type ConditionalKeys<Base, Condition> =
{
// Map through all the keys of the given base type.
[Key in keyof Base]-?:
// Pick only keys with types extending the given `Condition` type.
Base[Key] extends Condition
// Retain this key
// If the value for the key extends never, only include it if `Condition` also extends never
? IfNever<Base[Key], IfNever<Condition, Key, never>, Key>
// Discard this key since the condition fails.
: never;
// Convert the produced object into a union type of the keys which passed the conditional test.
}[keyof Base];
Version data entries
28 entries across 28 versions & 2 rubygems