Improved error output for FormPath

Author: ciscoheat

CHANGELOG.md

@@ -5,6 +5,12 @@ Headlines: Added, Changed, Deprecated, Removed, Fixed, Security
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- Fixed type inference for `FormPath` with nested arrays. Error output improved as well.
+
 ## [2.11.0] - 2024-03-22
 
 ### Added

src/lib/stringPath.ts

@@ -24,28 +24,32 @@ type BuiltInObjects = Date | Set<unknown> | File;
 /**
  * Lists all paths in an object as string accessors.
  */
-export type FormPath<T extends object, Type = any> = string &
-	StringPath<T, { filter: 'all'; objAppend: never; path: ''; type: Type }>;
+export type FormPath<T extends object, Type = any> = string & T extends any
+	? StringPath<T, { filter: 'all'; objAppend: never; path: ''; type: Type }>
+	: never;
 
 /**
  * List paths in an object as string accessors, but only with non-objects as accessible properties.
  * Similar to the leaves in a node tree, if you look at the object as a tree structure.
  */
-export type FormPathLeaves<T extends object, Type = any> = string &
-	StringPath<T, { filter: 'leaves'; objAppend: never; path: ''; type: Type }>;
+export type FormPathLeaves<T extends object, Type = any> = string & T extends any
+	? StringPath<T, { filter: 'leaves'; objAppend: never; path: ''; type: Type }>
+	: never;
 
 /**
  * List paths in an object as string accessors, but only with non-objects as accessible properties.
  * Also includes the _errors field for objects and arrays.
  */
-export type FormPathLeavesWithErrors<T extends object, Type = any> = string &
-	StringPath<T, { filter: 'leaves'; objAppend: '_errors'; path: ''; type: Type }>;
+export type FormPathLeavesWithErrors<T extends object, Type = any> = string & T extends any
+	? StringPath<T, { filter: 'leaves'; objAppend: '_errors'; path: ''; type: Type }>
+	: never;
 
 /**
  * List all arrays in an object as string accessors.
  */
-export type FormPathArrays<T extends object, Type = any> = string &
-	StringPath<T, { filter: 'arrays'; objAppend: never; path: ''; type: Type }>;
+export type FormPathArrays<T extends object, Type = any> = string & T extends any
+	? StringPath<T, { filter: 'arrays'; objAppend: never; path: ''; type: Type }>
+	: never;
 
 type Concat<
 	Path extends string,

src/tests/legacy/paths.test-d.ts

@@ -324,6 +324,18 @@ test('FormPath with type narrowing, arrays', () => {
 	const i2: NameArrays = 'tags';
 });
 
+test('FormPath with type narrowing, arrays 2', () => {
+	type NameArrays = FormPath<FormData, string[]>;
+
+	const t1: NameArrays = 'field1';
+	const t2: NameArrays = 'field3.nestedArray[3].innerArray';
+
+	// @ts-expect-error incorrect path
+	const i1: NameArrays = 'field1[2].id';
+	// @ts-expect-error incorrect path
+	const i2: NameArrays = 'field3.nestedArray[2]';
+});
+
 test('FormPath with type narrowing, union', () => {
 	type NameArrays = FormPath<Obj, string | number>;