type-fest/test-d/simplify.ts

import {expectAssignable, expectNotAssignable, expectType} from 'tsd';
import type {Simplify} from '../index';

type PositionProps = {
	top: number;
	left: number;
};

type SizeProps = {
	width: number;
	height: number;
};

// Flatten the type output to improve type hints shown in editors.
const flattenProps = {top: 120, left: 240, width: 480, height: 600};
expectType<Simplify<PositionProps & SizeProps>>(flattenProps);

// eslint-disable-next-line @typescript-eslint/consistent-type-definitions
interface SomeInterface {
	foo: number;
	bar?: string;
	baz: number | undefined;
}

type SomeInterfaceAsTypeWrittenByHand = {
	foo: number;
	bar?: string;
	baz: number | undefined;
};

const valueAsLiteral = {foo: 123, bar: 'hello', baz: 456};
const valueAsSimplifiedInterface: Simplify<SomeInterface> = valueAsLiteral;
const valueAsInterface: SomeInterface = valueAsLiteral;

declare const a: Simplify<SomeInterface>;
expectType<SomeInterfaceAsTypeWrittenByHand>(a);

// Interface is assignable to its Simplified type (created with Simplify, and by hand)
expectType<Simplify<SomeInterface>>(valueAsInterface);
expectType<SomeInterfaceAsTypeWrittenByHand>(valueAsInterface);

// The following demonstrates one reason a type may be preferred over an interface is that it can be assigned to alternate types. In this example the interface cannot be because it is not sealed and elsewhere a non-string property could be added.
expectAssignable<Record<string, unknown>>(valueAsLiteral);
expectAssignable<Record<string, unknown>>(valueAsSimplifiedInterface);
expectNotAssignable<Record<string, unknown>>(valueAsInterface); // Index signature is missing in interface

// The following tests should be fixed once we have determined the cause of the bug reported in https://github.com/sindresorhus/type-fest/issues/436

type SomeFunction = (type: string) => string;
type SimplifiedFunction = Simplify<SomeFunction>; // Return '{}' expected 'SomeFunction'

declare const someFunction: SimplifiedFunction;

expectNotAssignable<SomeFunction>(someFunction);

// // 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());