Add SingleKeyObject and IfEmptyObject types (#849)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

index.d.ts

@@ -9,6 +9,7 @@ export type {KeysOfUnion} from './source/keys-of-union';
 export type {DistributedOmit} from './source/distributed-omit';
 export type {DistributedPick} from './source/distributed-pick';
 export type {EmptyObject, IsEmptyObject} from './source/empty-object';
+export type {IfEmptyObject} from './source/if-empty-object';
 export type {NonEmptyObject} from './source/non-empty-object';
 export type {UnknownRecord} from './source/unknown-record';
 export type {UnknownArray} from './source/unknown-array';
@@ -23,6 +24,7 @@ export type {RequireAtLeastOne} from './source/require-at-least-one';
 export type {RequireExactlyOne} from './source/require-exactly-one';
 export type {RequireAllOrNone} from './source/require-all-or-none';
 export type {RequireOneOrNone} from './source/require-one-or-none';
+export type {SingleKeyObject} from './source/single-key-object';
 export type {OmitIndexSignature} from './source/omit-index-signature';
 export type {PickIndexSignature} from './source/pick-index-signature';
 export type {PartialDeep, PartialDeepOptions} from './source/partial-deep';

readme.md

@@ -110,7 +110,6 @@ Click the type names for complete docs.
 ### Utilities
 
 - [`EmptyObject`](source/empty-object.d.ts) - Represents a strictly empty plain object, the `{}` value.
-- [`IsEmptyObject`](source/empty-object.d.ts) - Returns a `boolean` for whether the type is strictly equal to an empty plain object, the `{}` value.
 - [`NonEmptyObject`](source/non-empty-object.d.ts) - Represents an object with at least 1 non-optional key.
 - [`UnknownRecord`](source/unknown-record.d.ts) - Represents an object with `unknown` value. You probably want this instead of `{}`.
 - [`UnknownArray`](source/unknown-array.d.ts) - Represents an array with `unknown` value.
@@ -125,6 +124,7 @@ Click the type names for complete docs.
 - [`RequireExactlyOne`](source/require-exactly-one.d.ts) - Create a type that requires exactly a single key of the given keys and disallows more.
 - [`RequireAllOrNone`](source/require-all-or-none.d.ts) - Create a type that requires all of the given keys or none of the given keys.
 - [`RequireOneOrNone`](source/require-one-or-none.d.ts) - Create a type that requires exactly a single key of the given keys and disallows more, or none of the given keys.
+- [`SingleKeyObject`](source/single-key-object.d.ts) - Create a type that only accepts an object with a single key.
 - [`RequiredDeep`](source/required-deep.d.ts) - Create a deeply required version of another type. Use [`Required<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#requiredtype) if you only need one level deep.
 - [`PickDeep`](source/pick-deep.d.ts) - Pick properties from a deeply-nested object. Use [`Pick<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#picktype-keys) if you only need one level deep.
 - [`OmitDeep`](source/omit-deep.d.ts) - Omit properties from a deeply-nested object. Use [`Omit<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#omittype-keys) if you only need one level deep.
@@ -220,6 +220,7 @@ type ShouldBeNever = IfAny<'not any', 'not never', 'never'>;
 - [`IsAny`](source/is-any.d.ts) - Returns a boolean for whether the given type is `any`. (Conditional version: [`IfAny`](source/if-any.d.ts).)
 - [`IsNever`](source/is-never.d.ts) - Returns a boolean for whether the given type is `never`. (Conditional version: [`IfNever`](source/if-never.d.ts).)
 - [`IsUnknown`](source/is-unknown.d.ts) - Returns a boolean for whether the given type is `unknown`. (Conditional version: [`IfUnknown`](source/if-unknown.d.ts).)
+- [`IsEmptyObject`](source/empty-object.d.ts) - Returns a boolean for whether the type is strictly equal to an empty plain object, the `{}` value. (Conditional version: [`IfEmptyObject`](source/if-empty-object.d.ts))
 
 ### JSON
 

source/if-empty-object.d.ts

@@ -0,0 +1,26 @@
+import type {IsEmptyObject} from './empty-object';
+
+/**
+An if-else-like type that resolves depending on whether the given type is `{}`.
+
+@see {@link IsEmptyObject}
+
+@example
+```
+import type {IfEmptyObject} from 'type-fest';
+
+type ShouldBeTrue = IfEmptyObject<{}>;
+//=> true
+
+type ShouldBeBar = IfEmptyObject<{key: any}, 'foo', 'bar'>;
+//=> 'bar'
+```
+
+@category Type Guard
+@category Utilities
+*/
+export type IfEmptyObject<
+	T,
+	TypeIfEmptyObject = true,
+	TypeIfNotEmptyObject = false,
+> = IsEmptyObject<T> extends true ? TypeIfEmptyObject : TypeIfNotEmptyObject;

source/single-key-object.d.ts

@@ -0,0 +1,29 @@
+import type {IfEmptyObject} from '../index';
+import type {IsUnion} from './internal';
+
+/**
+Create a type that only accepts an object with a single key.
+
+@example
+```
+import type {SingleKeyObject} from 'type-fest';
+
+const someFunction = <T>(parameter: SingleKeyObject<T>) => {};
+
+someFunction({
+	value: true
+});
+
+someFunction({
+	value: true,
+	otherKey: true
+});
+// Error: Argument of type '{value: boolean; otherKey: boolean}' is not assignable to parameter of type 'never'.ts(2345)
+```
+
+@category Object
+*/
+export type SingleKeyObject<ObjectType> =
+	IsUnion<keyof ObjectType> extends true
+		? never
+		: IfEmptyObject<ObjectType, never, ObjectType>;

test-d/single-key-object.ts

@@ -0,0 +1,15 @@
+import {expectNever, expectError, expectAssignable} from 'tsd';
+import type {SingleKeyObject} from '../index';
+
+const test = <T>(_: SingleKeyObject<T>): void => {}; // eslint-disable-line @typescript-eslint/no-empty-function
+
+test({key: 'value'});
+
+expectError(test({}));
+expectError(test({key: 'value', otherKey: 'other value'}));
+
+declare const validObject: SingleKeyObject<{key: string}>;
+expectAssignable<{key: string}>(validObject);
+
+declare const invalidObject: SingleKeyObject<{key1: string; key2: number}>;
+expectNever(invalidObject);