type-fest/source/paths.d.ts

A collection of essential TypeScript types

import type {StaticPartOfArray, VariablePartOfArray, NonRecursiveType, ToString} from './internal';
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 {Sum} from './sum';
import type {LessThan} from './less-than';

/**
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> = Paths_<T>;

type Paths_<T, Depth extends number = 0> =
	T extends NonRecursiveType | ReadonlyMap<unknown, unknown> | ReadonlySet<unknown>
		? never
		: 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<StaticPartOfArray<T>, Depth>
					| InternalPaths<Array<VariablePartOfArray<T>[number]>, Depth>
					: InternalPaths<T, Depth>
				: T extends object
					? InternalPaths<T, Depth>
					: never;

export type InternalPaths<_T, Depth extends number = 0, T = Required<_T>> =
	T extends EmptyObject | readonly []
		? never
		: {
			[Key in keyof T]:
			Key extends string | number // Limit `Key` to string or number.
				// If `Key` is a number, return `Key | `${Key}``, because both `array[0]` and `array['0']` work.
				?
				| Key
				| ToString<Key>
				| (
					LessThan<Depth, 15> extends true // Limit the depth to prevent infinite recursion
						? IsNever<Paths_<T[Key], Sum<Depth, 1>>> extends false
							? `${Key}.${Paths_<T[Key], Sum<Depth, 1>>}`
							: never
						: never
				)
				: never
		}[keyof T & (T extends UnknownArray ? number : unknown)];