Add Paths type (#741)

Author: Emiyaaaaa

index.d.ts

@@ -103,6 +103,7 @@ export type {IfUnknown} from './source/if-unknown';
 export type {ArrayIndices} from './source/array-indices';
 export type {ArrayValues} from './source/array-values';
 export type {SetFieldType} from './source/set-field-type';
+export type {Paths} from './source/paths';
 
 // Template literal types
 export type {CamelCase} from './source/camel-case';

readme.md

@@ -177,6 +177,7 @@ Click the type names for complete docs.
 - [`ArrayIndices`](source/array-indices.d.ts) - Provides valid indices for a constant array or tuple.
 - [`ArrayValues`](source/array-values.d.ts) - Provides all values for a constant array or tuple.
 - [`SetFieldType`](source/set-field-type.d.ts) - Create a type that changes the type of the given keys.
+- [`Paths`](source/paths.d.ts) - Generate a union of all possible paths to properties in the given object.
 
 ### Type Guard
 

source/paths.d.ts

@@ -0,0 +1,108 @@
+import type {EmptyObject} from './empty-object';
+import type {IsAny} from './is-any';
+import type {IsNever} from './is-never';
+import type {UnknownArray} from './unknown-array';
+import type {UnknownRecord} from './unknown-record';
+
+type ToString<T> = T extends string | number ? `${T}` : never;
+
+/**
+Return the part of the given array with a fixed index.
+
+@example
+```
+type A = [string, number, boolean, ...string[]];
+type B = FilterFixedIndexArray<A>;
+//=> [string, number, boolean]
+```
+*/
+type FilterFixedIndexArray<T extends UnknownArray, Result extends UnknownArray = []> =
+	number extends T['length'] ?
+		T extends readonly [infer U, ...infer V]
+			? FilterFixedIndexArray<V, [...Result, U]>
+			: Result
+		: T;
+
+/**
+Return the part of the given array with a non-fixed index.
+
+@example
+```
+type A = [string, number, boolean, ...string[]];
+type B = FilterNotFixedIndexArray<A>;
+//=> string[]
+```
+*/
+type FilterNotFixedIndexArray<T extends UnknownArray> =
+T extends readonly [...FilterFixedIndexArray<T>, ...infer U]
+	? U
+	: [];
+
+/**
+Generate a union of all possible paths to properties in the given object.
+
+It also works with arrays.
+
+Use-case: You want a type-safe way to access deeply nested properties in an object.
+
+@example
+```
+import type {Paths} from 'type-fest';
+
+type Project = {
+	filename: string;
+	listA: string[];
+	listB: [{filename: string}];
+	folder: {
+		subfolder: {
+			filename: string;
+		};
+	};
+};
+
+type ProjectPaths = Paths<Project>;
+//=> 'filename' | 'listA' | 'listB' | 'folder' | `listA.${number}` | 'listB.0' | 'listB.0.filename' | 'folder.subfolder' | 'folder.subfolder.filename'
+
+declare function open<Path extends ProjectPaths>(path: Path): void;
+
+open('filename'); // Pass
+open('folder.subfolder'); // Pass
+open('folder.subfolder.filename'); // Pass
+open('foo'); // TypeError
+
+// Also works with arrays
+open('listA.1'); // Pass
+open('listB.0'); // Pass
+open('listB.1'); // TypeError. Because listB only has one element.
+```
+
+@category Object
+@category Array
+*/
+export type Paths<T extends UnknownRecord | UnknownArray> =
+	IsAny<T> extends true
+		? never
+		: T extends UnknownArray
+			? number extends T['length']
+				// We need to handle the fixed and non-fixed index part of the array separately.
+				? InternalPaths<FilterFixedIndexArray<T>>
+				| InternalPaths<Array<FilterNotFixedIndexArray<T>[number]>>
+				: InternalPaths<T>
+			: InternalPaths<T>;
+
+export type InternalPaths<_T extends UnknownRecord | UnknownArray, T = Required<_T>> =
+	T extends EmptyObject | readonly []
+		? never
+		: {
+			[Key in keyof T]:
+			Key extends string | number // Limit `Key` to string or number.
+				? T[Key] extends UnknownRecord | UnknownArray
+					? (
+						IsNever<Paths<T[Key]>> extends false
+							// If `Key` is a number, return `Key | `${Key}``, because both `array[0]` and `array['0']` do not work.
+							? Key | ToString<Key> | `${Key}.${Paths<T[Key]>}`
+							: Key | ToString<Key>
+					)
+					: Key | ToString<Key>
+				: never
+		}[keyof T & (T extends UnknownArray ? number : unknown)];

test-d/paths.ts

@@ -0,0 +1,75 @@
+import {expectType} from 'tsd';
+import type {Paths} from '../index';
+
+declare const normal: Paths<{foo: string}>;
+expectType<'foo'>(normal);
+
+type DeepObject = {
+	a: {
+		b: {
+			c: {
+				d: string;
+			};
+		};
+		b2: number[];
+		b3: boolean;
+	};
+};
+declare const deepObject: Paths<DeepObject>;
+expectType<'a' | 'a.b' | 'a.b2' | 'a.b3' | 'a.b.c' | 'a.b.c.d' | `a.b2.${number}`>(deepObject);
+
+declare const emptyObject: Paths<{}>;
+expectType<never>(emptyObject);
+
+declare const emptyArray: Paths<[]>;
+expectType<never>(emptyArray);
+
+declare const symbol: Paths<{[Symbol.iterator]: string}>;
+expectType<never>(symbol);
+
+declare const never: Paths<never>;
+expectType<never>(never);
+
+declare const date: Paths<{foo: Date}>;
+expectType<'foo'>(date);
+
+declare const mixed: Paths<{foo: boolean} | {bar: string}>;
+expectType<'foo' | 'bar'>(mixed);
+
+declare const array: Paths<Array<{foo: string}>>;
+expectType<number | `${number}` | `${number}.foo`>(array);
+
+declare const tuple: Paths<[{foo: string}]>;
+expectType<'0' | '0.foo'>(tuple);
+
+declare const deeplist: Paths<{foo: Array<{bar: boolean[]}>}>;
+expectType<'foo' | `foo.${number}` | `foo.${number}.bar` | `foo.${number}.bar.${number}`>(deeplist);
+
+declare const readonly: Paths<{foo: Readonly<{bar: string}>}>;
+expectType<'foo' | 'foo.bar'>(readonly);
+
+declare const readonlyArray: Paths<{foo: readonly string[]}>;
+expectType<'foo' | `foo.${number}`>(readonlyArray);
+
+declare const optional: Paths<{foo?: {bar?: number}}>;
+expectType<'foo' | 'foo.bar'>(optional);
+
+declare const record: Paths<Record<'a', any>>;
+expectType<'a'>(record);
+
+declare const record2: Paths<Record<1, unknown>>;
+expectType<1 | '1'>(record2);
+
+// Test for unknown length array
+declare const trailingSpreadTuple: Paths<[{a: string}, ...Array<{b: number}>]>;
+expectType<number | `${number}` | '0.a' | `${number}.b`>(trailingSpreadTuple);
+
+declare const trailingSpreadTuple1: Paths<[{a: string}, {b: number}, ...Array<{c: number}>]>;
+expectType<number | `${number}` | '0.a' | `${number}.b`>(trailingSpreadTuple);
+expectType<number | `${number}` | '0.a' | '1.b' | `${number}.c`>(trailingSpreadTuple1);
+
+declare const leadingSpreadTuple: Paths<[...Array<{a: string}>, {b: number}]>;
+expectType<number | `${number}` | `${number}.b` | `${number}.a`>(leadingSpreadTuple);
+
+declare const leadingSpreadTuple1: Paths<[...Array<{a: string}>, {b: number}, {c: number}]>;
+expectType<number | `${number}` | `${number}.b` | `${number}.c` | `${number}.a`>(leadingSpreadTuple1);