Add Filter type

source/array-filter.d.ts

@@ -0,0 +1,68 @@
+import type {OptionalKeysOf} from './optional-keys-of.d.ts';
+import type {IsTruthy, Extends} from './internal/type.d.ts';
+import type {UnknownArray} from './unknown-array.d.ts';
+import type {CleanEmpty} from './internal/array.d.ts';
+import type {IsAny} from './is-any.d.ts';
+
+/**
+Returns a boolean for whether a value `T` extends the filtering type `U`.
+
+If `U` is `Boolean`, it checks whether `T` is `truthy` like {@link Boolean `Boolean(T)`} does.
+
+Otherwise, it uses {@link Extends `Extends<T, U, S>`} to check if `T extends U` with strict or loose mode.
+*/
+type FilterType<T, U, S extends boolean> =
+	Boolean extends U
+		? IsTruthy<T>
+		: Extends<T, U, S>;
+
+/**
+Determines whether the array `V` should be kept based on the boolean type `T`.
+*/
+type IfFilter<T extends boolean, V extends UnknownArray> = [T] extends [true] ? V : [];
+
+/**
+Filters elements from an `Array_` based on whether they match the given `Type`.
+
+If `Type` is `Boolean`, it filters out `falsy` values like {@link Boolean `Boolean(T)`} does.
+
+Strict controls whether strict or loose type comparison is used (defaults to loose).
+*/
+export type ArrayFilter<
+	Array_ extends UnknownArray, Type,
+	Strict extends boolean = false,
+> = IsAny<Array_> extends true ? []
+	: CleanEmpty<_ArrayFilter<Array_, Type, Strict>>;
+
+/**
+Internal implementation of {@link ArrayFilter}.
+
+Iterates through the array and includes elements in the accumulator if they pass `FilterType`.
+*/
+type _ArrayFilter<
+	Array_ extends UnknownArray, Type,
+	Strict extends boolean = false,
+	Head extends any[] = [],
+	Tail extends any[] = [],
+> =
+	keyof Array_ & `${number}` extends never // Is `Array_` leading a rest element or empty
+		? Array_ extends readonly [...infer Rest, infer Last]
+			? _ArrayFilter<Rest, Type, Strict, Head, [
+				...IfFilter<FilterType<Last, Type, Strict>, [Last]>,
+				...Tail,
+			]>
+			: [
+				...Head,
+				...IfFilter<FilterType<Array_[number], Type, Strict>, Array_>,
+				...Tail,
+			]
+		: Array_ extends readonly [(infer First)?, ...infer Rest]
+			? _ArrayFilter<Rest, Type, Strict, [
+				...Head,
+				...IfFilter<FilterType<First, Type, Strict>,
+					'0' extends OptionalKeysOf<Array_> // TODO: replace with `IsOptionalKeyOf`.
+						? [First?]
+						: [First]
+				>,
+			], Tail>
+			: never;

source/internal/array.d.ts

@@ -1,7 +1,10 @@
 import type {If} from '../if.d.ts';
+import type {IsAny} from '../is-any.d.ts';
 import type {IsNever} from '../is-never.d.ts';
-import type {OptionalKeysOf} from '../optional-keys-of.d.ts';
+import type {IsUnion} from '../is-union.d.ts';
+import type {EmptyObject} from '../empty-object.d.ts';
 import type {UnknownArray} from '../unknown-array.d.ts';
+import type {OptionalKeysOf} from '../optional-keys-of.d.ts';
 import type {IsExactOptionalPropertyTypesEnabled, IfNotAnyOrNever} from './type.d.ts';
 
 /**
@@ -96,6 +99,35 @@ Returns whether the given array `T` is readonly.
 */
 export type IsArrayReadonly<T extends UnknownArray> = If<IsNever<T>, false, T extends unknown[] ? false : true>;
 
+/**
+Represents an empty array, the `[]` or `readonly []` value.
+*/
+export type EmptyArray = readonly [] | []; // The extra `[]` is just to prevent TS from expanding the type.
+
+/**
+Returns a `boolean` for whether the type is an empty array, the `[]` or `readonly []` value.
+@example
+```
+import type {IsEmptyArray} from 'type-fest';
+type Pass1 = IsEmptyArray<[]>;
+//=> true
+type Pass2 = IsEmptyArray<readonly []>;
+//=> true
+type Fail1 = IsEmptyArray<[0]>;
+//=> false
+type Fail2 = IsEmptyArray<[0?]>;
+//=> false
+type Fail3 = IsEmptyArray<...string[]>;
+//=> false
+```
+@see EmptyArray
+@category Array
+*/
+export type IsEmptyArray<T> =
+	IsNever<T> extends true ? false
+		: T extends EmptyArray ? true
+			: false;
+
 /**
 Transforms a tuple type by replacing it's rest element with a single element that has the same type as the rest element, while keeping all the non-rest elements intact.
 
@@ -156,3 +188,25 @@ type _CollapseRestElement<
 				>
 				: never // Should never happen, since `[(infer First)?, ...infer Rest]` is a top-type for arrays.
 		: never; // Should never happen
+
+/**
+Cleans any extra empty arrays/objects from a union.
+
+@example
+```
+type T1 = CleanEmpty<[number] | []>
+//=> [number]
+
+type T2 = CleanEmpty<[number, string?] | [never] | []>
+//=> [number, string?] | [never]
+
+type T3 = CleanEmpty<[]>
+//=> []
+```
+
+@category Utilities
+*/
+export type CleanEmpty<T> =
+	Exclude<T, EmptyArray | EmptyObject> extends infer U
+		? IsNever<U> extends true ? T : U
+		: never;

source/internal/type.d.ts

@@ -2,6 +2,7 @@ import type {If} from '../if.d.ts';
 import type {IsAny} from '../is-any.d.ts';
 import type {IsNever} from '../is-never.d.ts';
 import type {Primitive} from '../primitive.d.ts';
+import type {ExtendsStrict} from '../extends-strict.d.ts';
 
 /**
 Matches any primitive, `void`, `Date`, or `RegExp` value.
@@ -100,9 +101,40 @@ type C = IfNotAnyOrNever<never, 'VALID', 'IS_ANY', 'IS_NEVER'>;
 export type IfNotAnyOrNever<T, IfNotAnyOrNever, IfAny = any, IfNever = never> =
 	If<IsAny<T>, IfAny, If<IsNever<T>, IfNever, IfNotAnyOrNever>>;
 
-/*
+/**
 Indicates the value of `exactOptionalPropertyTypes` compiler option.
 */
 export type IsExactOptionalPropertyTypesEnabled = [(string | undefined)?] extends [string?]
 	? false
 	: true;
+
+/**
+Evaluates whether type `T extends U`, using either strict or loose comparison.
+
+ - Strict mode, {@link ExtendsStrict `ExtendsStrict<T, U>`} is used.
+ - Loose mode, {@link ExtendsLoose `ExtendsLoose<T, U>`} is used.
+*/
+export type Extends<T, U, S extends boolean = false> = {
+	true: ExtendsStrict<T, U>;
+	false: ExtendsLoose<T, U>;
+}[`${S}`];
+
+/**
+Performs a loose type comparison: checks if wheither of the members in `T` extends `U`.
+
+This is useful when needing to know if `T extends U` without distributing `T` in Main type
+*/
+export type ExtendsLoose<T, U> = IsNotFalse<T extends U ? true : false>;
+
+/**
+A union of `falsy` types in JS.
+*/
+export type Falsy = false | 0 | '' | null | undefined; // `| never`
+
+/**
+Checks if `T` is {@link Falsy `falsy`} similar to `Boolean(T)`.
+*/
+export type IsTruthy<T> =
+	IsNever<T> extends true ? false
+		: T extends Falsy ? false
+			: true; // ? Should this get exposed publicly

source/is-union.d.ts

@@ -19,8 +19,7 @@ export type IsUnion<T> = InternalIsUnion<T>;
 /**
 The actual implementation of `IsUnion`.
 */
-type InternalIsUnion<T, U = T> =
-(
+type InternalIsUnion<T, U = T> = (
 	IsNever<T> extends true
 		? false
 		: T extends any

source/object-filter.d.ts

@@ -0,0 +1,18 @@
+import type {UnknownRecord} from './unknown-record.d.ts';
+import type {CleanEmpty} from './internal/array.d.ts';
+import type {FilterType} from './array-filter.d.ts';
+import type {Simplify} from './simplify.d.ts';
+
+/**
+Filters properties from an object where the property value matches the given type.
+
+If `Type` is `Boolean`, it filters out `falsy` values like `Boolean(T)` does.
+
+Strict controls whether strict or loose type comparison is used (defaults to loose).
+*/
+export type ObjectFilter<
+	Object_ extends UnknownRecord, Type,
+	Strict extends boolean = false,
+> = CleanEmpty<Simplify<{
+	[K in keyof Object_ as FilterType<Object_[K], Type, Strict> extends true ? K : never]: Object_[K]
+}>>;

test-d/array-filter.ts

@@ -0,0 +1,115 @@
+import {expectType} from 'tsd';
+import type {ArrayFilter} from '../source/array-filter.d.ts';
+
+// Basic loose filtering
+expectType<ArrayFilter<[1, 2, 3, 3, 4], 3, true>>([3, 3]);
+expectType<ArrayFilter<[1, '2', 3, 'foo', false], number>>([1, 3]);
+expectType<ArrayFilter<[1, '2', 3, 'foo', false], string>>(['2', 'foo']);
+expectType<ArrayFilter<[1, '2', 3, 'foo', false], string | number>>([1, '2', 3, 'foo']);
+expectType<ArrayFilter<['foo', 'baz', 'foo', 'foo'], 'foo', true>>(['foo', 'foo', 'foo']);
+expectType<ArrayFilter<[1, '2', 3, 'foo', false], string | number, true>>([1, '2', 3, 'foo']);
+expectType<ArrayFilter<['1', '2', 3, 4, 'foo'], `${number}`>>(['1', '2']);
+expectType<ArrayFilter<[true, false, true, 0, 1], boolean>>([true, false, true]);
+expectType<ArrayFilter<[true, false, true, 0, 1], true>>([true, true]);
+
+// Filtering Boolean (keep truthy values)
+expectType<ArrayFilter<[true, false, boolean, 0, 1], Boolean>>([true, 1]);
+expectType<ArrayFilter<[true, false, boolean, 0, 1], Boolean, true>>([true, 1]);
+expectType<ArrayFilter<[0, '', false, null, undefined, 'ok', 42], Boolean>>(['ok', 42]);
+expectType<ArrayFilter<[true, false, 0, 1, '', 'text', null, undefined], Boolean>>([true, 1, 'text']);
+
+// Filtering objects
+type Object1 = {a: number};
+type Object2 = {b: string};
+expectType<ArrayFilter<[Object1, Object2, Object1 & Object2], Object1>>({} as [Object1, Object1 & Object2]);
+expectType<ArrayFilter<[Object1, Object2, Object1 & Object2], Object1, true>>({} as [Object1, Object1 & Object2]);
+
+// Loose filtering by boolean or number
+expectType<ArrayFilter<[true, 0, 1, false, 'no'], boolean | number>>([true, 0, 1, false]);
+
+// Filtering array containing null | undefined | string
+expectType<ArrayFilter<[null, undefined, 'foo', ''], string>>(['foo', '']);
+
+// Filtering with unknown type (should keep everything)
+expectType<ArrayFilter<[1, 'a', true], unknown>>([1, 'a', true]);
+
+// Filtering with any type (should keep everything)
+expectType<ArrayFilter<[1, 'a', true], any>>([1, 'a', true]);
+
+// Filtering with never type (should remove everything)
+expectType<ArrayFilter<[1, 2, 3], never>>([]);
+// ? Shoud we change this behavior ?
+
+// Filtering array of arrays by array type
+expectType<ArrayFilter<[[number], string[], number[]], number[]>>([[1], [2, 3]]);
+
+// Filtering by a union including literal and broader type
+expectType<ArrayFilter<[1, 2, 3, 'foo', 'bar'], 1 | string>>([1, 'foo', 'bar']);
+
+// Filtering complex nested union types
+type Nested = {x: string} | {y: number} | null;
+expectType<ArrayFilter<[ {x: 'a'}, {y: 2}, null, {z: true} ], Nested>>([{x: 'a'}, {y: 2}, null]);
+
+// Filtering with boolean type but array has no boolean values
+expectType<ArrayFilter<[1, 2, 3], Boolean>>([1, 2, 3]);
+
+// Filtering with boolean type but array has falsy values
+expectType<ArrayFilter<[0, '', false, null, undefined], Boolean>>([]);
+
+// Filtering string literals with template literal union
+expectType<ArrayFilter<['foo1', 'bar2', 'foo3'], `foo${number}`>>(['foo1', 'foo3']);
+
+// Filtering with `Boolean` type but including custom objects with truthy/falsy behavior
+class Foo {}
+expectType<ArrayFilter<[typeof Foo, {}, null, undefined], Boolean>>([Foo, {}]);
+
+// Filtering with strict = true and union including literals and primitives
+expectType<ArrayFilter<[1, '1', 2, '2', true, false], number | `${number}`, true>>([1, '1', 2, '2']);
+
+// Filtering falsy values mixed with ({} | [] is truthy)
+expectType<ArrayFilter<[false, 0, '', null, undefined, {}, []], Boolean>>([{}, []]);
+
+// Filtering with `true` literal (strict) but array contains boolean and number
+expectType<ArrayFilter<[true, false, 1, 0], true, true>>([true]);
+
+// Filtering empty string literal type with strict mode
+expectType<ArrayFilter<['', 'non-empty'], '', true>>(['']);
+
+// Filtering with loose mode for literal union type and matching subset
+expectType<ArrayFilter<[1, 2, 3, 4, 5], 2 | 3>>([2, 3]);
+
+// Filtering tuples with mixed optional and required elements
+type Tuple = [string, number?, boolean?];
+expectType<ArrayFilter<Tuple, number>>({} as [number?]);
+expectType<ArrayFilter<Tuple, string | boolean>>({} as [string, boolean?]);
+
+// Rest elements
+expectType<ArrayFilter<['f', ...string[], 's'], string>>({} as ['f', ...string[], 's']);
+expectType<ArrayFilter<['f', ...string[], 's'], 'f' | 's'>>({} as ['f', 's']);
+expectType<ArrayFilter<[string, ...string[]], string>>({} as [string, ...string[]]);
+expectType<ArrayFilter<[string, ...string[], number], string>>({} as [string, ...string[]]);
+
+// Rest and Optional
+expectType<ArrayFilter<[true, number?, ...string[]], string>>({} as string[]);
+expectType<ArrayFilter<[true, number?, ...string[]], number | string>>({} as [number?, ...string[]]);
+expectType<ArrayFilter<[string?, ...string[]], number | string>>({} as [string?, ...string[]]);
+
+// Union
+expectType<ArrayFilter<[1, '2', 3, false] | ['1', 2, '3', true], number>>({} as [1, 3] | [2]);
+expectType<ArrayFilter<[1, '2', 3, false] | ['1', 2, '3', true], string>>({} as ['2'] | ['1', '3']);
+expectType<ArrayFilter<[true, number?, ...string[]] | [false?, ...Array<'foo'>], string>>({} as string[] | Array<'foo'>);
+expectType<ArrayFilter<[true, number?, ...string[]] | [false?, ...Array<'foo'>], number>>({} as [number?]);
+
+// Edge cases
+expectType<ArrayFilter<any, any>>({} as []);
+expectType<ArrayFilter<any, never>>([]);
+expectType<ArrayFilter<any[], any>>({} as []);
+expectType<ArrayFilter<any[], never>>({} as any[]);
+expectType<ArrayFilter<never, any>>({} as never);
+expectType<ArrayFilter<never, never>>({} as never);
+
+expectType<ArrayFilter<[], number>>([]);
+expectType<ArrayFilter<[never, never], number>>([]);
+expectType<ArrayFilter<[never, never], never>>([]);
+expectType<ArrayFilter<[never, never], never>>([]);
+expectType<ArrayFilter<[never, never], never, true>>({} as [never, never]);