Make Simplify handle all types (#414)

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

index.d.ts

@@ -38,7 +38,7 @@ export {Entry} from './source/entry';
 export {Entries} from './source/entries';
 export {SetReturnType} from './source/set-return-type';
 export {Asyncify} from './source/asyncify';
-export {Simplify} from './source/simplify';
+export {Simplify, SimplifyOptions} from './source/simplify';
 export {Jsonify} from './source/jsonify';
 export {Schema} from './source/schema';
 export {LiteralToPrimitive} from './source/literal-to-primitive';

source/simplify.d.ts

@@ -1,3 +1,23 @@
+/**
+@see Simplify
+*/
+export interface SimplifyOptions {
+	/**
+	Do the simplification recursively.
+
+	@default false
+	*/
+	deep?: boolean;
+}
+
+// Flatten a type without worrying about the result.
+type Flatten<
+	AnyType,
+	Options extends SimplifyOptions = {},
+> = Options['deep'] extends true
+	? {[KeyType in keyof AnyType]: Simplify<AnyType[KeyType], Options>}
+	: {[KeyType in keyof AnyType]: AnyType[KeyType]};
+
 /**
 Useful to flatten the type output to improve type hints shown in editors. And also to transform an interface into a type to aide with assignability.
 
@@ -55,4 +75,9 @@ fn(someInterface as Simplify<SomeInterface>); // Good: transform an `interface`
 
 @category Object
 */
-export type Simplify<T> = {[KeyType in keyof T]: T[KeyType]};
+export type Simplify<
+	AnyType,
+	Options extends SimplifyOptions = {},
+> = Flatten<AnyType> extends AnyType
+	? Flatten<AnyType, Options>
+	: AnyType;

test-d/simplify.ts

@@ -16,15 +16,15 @@ const flattenProps = {top: 120, left: 240, width: 480, height: 600};
 expectType<Simplify<PositionProps & SizeProps>>(flattenProps);
 
 interface SomeInterface {
-  foo: number;
-  bar?: string;
-  baz: number | undefined;
+	foo: number;
+	bar?: string;
+	baz: number | undefined;
 }
 
 type SomeInterfaceAsTypeWrittenByHand = {
-  foo: number;
-  bar?: string;
-  baz: number | undefined;
+	foo: number;
+	bar?: string;
+	baz: number | undefined;
 };
 
 const valueAsLiteral = {foo: 123, bar: 'hello', baz: 456};
@@ -42,3 +42,45 @@ expectType<SomeInterfaceAsTypeWrittenByHand>(valueAsInterface);
 expectAssignable<Record<string, unknown>>(valueAsLiteral);
 expectAssignable<Record<string, unknown>>(valueAsSimplifiedInterface);
 expectNotAssignable<Record<string, unknown>>(valueAsInterface); // Index signature is missing in interface
+
+// Should return the original type if it is not simplifiable, like a function.
+type SomeFunction = (type: string) => string;
+expectType<Simplify<SomeFunction>>((type: string) => type);
+
+class SomeClass {
+	id: string;
+
+	private readonly code: number;
+
+	constructor() {
+		this.id = 'some-class';
+		this.code = 42;
+	}
+
+	someMethod() {
+		return this.code;
+	}
+}
+
+expectType<Simplify<SomeClass>>(new SomeClass());
+
+// Test deep option
+// This is mostly visual, move the mouse over "expectAssignable" to see the result.
+type PositionAndSize = PositionProps & SizeProps;
+
+interface Node {
+	parent: PositionAndSize;
+	child: {
+		parent: PositionAndSize;
+	};
+}
+
+const node = {
+	parent: flattenProps,
+	child: {
+		parent: flattenProps,
+	},
+};
+
+expectAssignable<Simplify<Node>>(node);
+expectAssignable<Simplify<Node, {deep: true}>>(node);