Add ConditionalSimplify and ConditionalSimplifyDeep types (#442)

Author: skarab42

source/conditional-simplify.d.ts

@@ -0,0 +1,32 @@
+/**
+Simplifies a type while including and/or excluding certain types from being simplified. Useful to improve type hints shown in editors. And also to transform an interface into a type to aide with assignability.
+
+This type is **experimental** and was introduced as a result of this {@link https://github.com/sindresorhus/type-fest/issues/436 issue}. It should be used with caution.
+
+@internal
+@experimental
+@see Simplify
+@category Object
+*/
+export type ConditionalSimplify<Type, ExcludeType = never, IncludeType = unknown> = Type extends ExcludeType
+	? Type
+	: Type extends IncludeType
+	? {[TypeKey in keyof Type]: Type[TypeKey]}
+	: Type;
+
+/**
+Recursively simplifies a type while including and/or excluding certain types from being simplified.
+
+This type is **experimental** and was introduced as a result of this {@link https://github.com/sindresorhus/type-fest/issues/436 issue}. It should be used with caution.
+
+See {@link ConditionalSimplify} for usages and examples.
+
+@internal
+@experimental
+@category Object
+*/
+export type ConditionalSimplifyDeep<Type, ExcludeType = never, IncludeType = unknown> = Type extends ExcludeType
+	? Type
+	: Type extends IncludeType
+	? {[TypeKey in keyof Type]: ConditionalSimplifyDeep<Type[TypeKey], ExcludeType, IncludeType>}
+	: Type;

test-d/conditional-simplify.ts

@@ -0,0 +1,81 @@
+import {expectError, expectType} from 'tsd';
+import type {ConditionalSimplify, ConditionalSimplifyDeep} from '../source/conditional-simplify';
+
+type Position = {top: number; left: number};
+type Size = {width: number; height: number};
+
+// In your editor, hovering over `PositionAndSizeSimplified` will show a simplified object with all the properties.
+type PositionAndSizeIntersection = Position & Size;
+type PositionAndSizeSimplified = ConditionalSimplify<PositionAndSizeIntersection>;
+
+const position = {top: 120, left: 240};
+const size = {width: 480, height: 600};
+const positionAndSize = {...position, ...size};
+expectType<PositionAndSizeSimplified>(positionAndSize);
+
+// Exclude function type to be simplified.
+type SomeFunction = (type: string) => string;
+type SimplifiedFunctionFail = ConditionalSimplify<SomeFunction>; // Return '{}'
+type SimplifiedFunctionPass = ConditionalSimplify<SomeFunction, Function>; // Return '(type: string) => string'
+
+declare const simplifiedFunctionFail: SimplifiedFunctionFail;
+declare const simplifiedFunctionPass: SimplifiedFunctionPass;
+
+expectError<SomeFunction>(simplifiedFunctionFail);
+expectType<SomeFunction>(simplifiedFunctionPass);
+
+// Should simplify interface deeply.
+interface SomeNode {
+  parent: PositionAndSizeIntersection;
+  childs: Array<{parent: PositionAndSizeIntersection}>;
+}
+
+// In your editor, hovering over `SomeNodeSimplified` will show a simplified object with all the properties.
+type SomeNodeSimplified = ConditionalSimplifyDeep<SomeNode>;
+
+const someNode = {parent: positionAndSize, childs: [{parent: positionAndSize}, {parent: positionAndSize}]};
+expectType<SomeNodeSimplified>(someNode);
+
+// Should simplify interface deeply excluding Function type.
+interface MovablePosition extends Position {
+  move(position: Position): Position;
+}
+
+interface MovableCollection {
+	position: MovablePosition;
+	top: {position: MovablePosition; size: Size};
+	left: {position: MovablePosition; size: Size};
+}
+
+type MovableNodeSimplifiedFail = ConditionalSimplifyDeep<MovableCollection>;
+type MovableNodeSimplifiedPass = ConditionalSimplifyDeep<MovableCollection, Function>;
+
+declare const movableNodeSimplifiedFail: MovableNodeSimplifiedFail;
+declare const movableNodeSimplifiedPass: MovableNodeSimplifiedPass;
+
+expectError<MovableCollection>(movableNodeSimplifiedFail);
+expectType<MovableCollection>(movableNodeSimplifiedPass);
+
+const movablePosition = {
+	top: 42,
+	left: 42,
+	move(position: Position) {
+		return position;
+	},
+};
+
+const movableNode = {
+	position: movablePosition,
+	top: {position: movablePosition, size},
+	left: {position: movablePosition, size},
+};
+
+expectType<MovableNodeSimplifiedPass>(movableNode);
+
+// Should exclude `Function` and `Size` type (mainly visual, mouse over the statement).
+type ExcludeFunctionAndSize1 = ConditionalSimplifyDeep<MovableCollection, Function | Size>;
+expectType<ExcludeFunctionAndSize1>(movableNode);
+
+// Same as above but using `IncludeType` parameter (mainly visual, mouse over the statement).
+type ExcludeFunctionAndSize2 = ConditionalSimplifyDeep<MovableCollection, Function, MovableCollection | Position>;
+expectType<ExcludeFunctionAndSize2>(movableNode);