feat: added WritableKeys/ReadonlyKeys mapped types

README.md

@@ -80,6 +80,8 @@ This gives you the power to prioritize our work and support project contributors
 * [`Subtract<T, T1>`](#subtractt-t1)
 * [`Overwrite<T, U>`](#overwritet-u)
 * [`Assign<T, U>`](#assignt-u)
+* [`ReadonlyKeys<T>`](#readonlykeyst)
+* [`WritableKeys<T>`](#writablekeyst)
 
 ## Mapped Types
 
@@ -399,6 +401,38 @@ type ExtendedProps = Assign<Props, NewProps>;
 
 [⇧ back to top](#operations-on-objects)
 
+### `ReadonlyKeys<T>`
+
+Get union type of keys that are readonly in object type `T`
+
+**Usage:**
+
+```ts
+import { ReadonlyKeys } from 'utility-types';
+
+type Props = { readonly foo: string; bar: number };
+type ReadonlyProps = ReadonlyKeys<Props>;
+// Expect: "foo"
+```
+
+[⇧ back to top](#operations-on-objects)
+
+### `WritableKeys<T>`
+
+Get union type of keys that are writable (not readonly) in object type `T`
+
+**Usage:**
+
+```ts
+import { WritableKeys } from 'utility-types';
+
+type Props = { readonly foo: string; bar: number };
+type WritableProps = WritableKeys<Props>;
+// Expect: "bar"
+```
+
+[⇧ back to top](#operations-on-objects)
+
 ---
 
 ## Mapped Types

src/__snapshots__/mapped-types.spec.ts.snap

@@ -104,6 +104,8 @@ exports[`PickByValue testType<PickByValue<Props, string | number>>() 1`] = `"Pic
 
 exports[`PromiseType testType<PromiseType<Promise<string>>>() 1`] = `"string"`;
 
+exports[`ReadonlyKeys testType<ReadonlyKeys<ReadWriteProps>>() 1`] = `"\\"a\\""`;
+
 exports[`SetComplement testType<SetComplement<'1' | '2' | '3', '2' | '3'>>() 1`] = `"\\"1\\""`;
 
 exports[`SetDifference testType<SetDifference<'1' | '2' | '3', '2' | '3' | '4'>>() 1`] = `"\\"1\\""`;
@@ -119,3 +121,5 @@ exports[`Subtract testType<Subtract<Props, DefaultProps>>() 1`] = `"Pick<Props,
 exports[`SymmetricDifference testType<SymmetricDifference<'1' | '2' | '3', '2' | '3' | '4'>>() 1`] = `"\\"1\\" | \\"4\\""`;
 
 exports[`Unionize testType<Unionize<Props>>() 1`] = `"{ name: string; } | { age: number; } | { visible: boolean; }"`;
+
+exports[`WritableKeys testType<WritableKeys<ReadWriteProps>>() 1`] = `"\\"b\\""`;

src/index.ts

@@ -37,6 +37,8 @@ export {
   Subtract,
   SymmetricDifference,
   Unionize,
+  WritableKeys,
+  ReadonlyKeys,
 } from './mapped-types';
 
 // deprecated

src/mapped-types.spec.snap.ts

@@ -21,6 +21,8 @@ import {
   DeepRequired,
   DeepNonNullable,
   DeepPartial,
+  WritableKeys,
+  ReadonlyKeys,
   _DeepNonNullableArray,
   _DeepNonNullableObject,
   _DeepReadonlyArray,
@@ -39,6 +41,7 @@ type Props = { name: string; age: number; visible: boolean };
 type DefaultProps = { age: number };
 type NewProps = { age: string; other: string };
 type MixedProps = { name: string; setName: (name: string) => void };
+type ReadWriteProps = { readonly a: number; b: string };
 
 /**
  * Tests
@@ -337,3 +340,15 @@ it('DeepPartial', () => {
   // @dts-jest:pass:snap -> string
   testType<ReturnType<NonNullable<typeof functionProp>>>();
 });
+
+// @dts-jest:group WritableKeys
+it('WritableKeys', () => {
+  // @dts-jest:pass:snap -> "b"
+  testType<WritableKeys<ReadWriteProps>>();
+});
+
+// @dts-jest:group ReadonlyKeys
+it('ReadonlyKeys', () => {
+  // @dts-jest:pass:snap -> "a"
+  testType<ReadonlyKeys<ReadWriteProps>>();
+});

src/mapped-types.spec.ts

@@ -21,6 +21,8 @@ import {
   DeepRequired,
   DeepNonNullable,
   DeepPartial,
+  WritableKeys,
+  ReadonlyKeys,
   _DeepNonNullableArray,
   _DeepNonNullableObject,
   _DeepReadonlyArray,
@@ -39,6 +41,7 @@ type Props = { name: string; age: number; visible: boolean };
 type DefaultProps = { age: number };
 type NewProps = { age: string; other: string };
 type MixedProps = { name: string; setName: (name: string) => void };
+type ReadWriteProps = { readonly a: number; b: string };
 
 /**
  * Tests
@@ -337,3 +340,15 @@ it('DeepPartial', () => {
   // @dts-jest:pass:snap
   testType<ReturnType<NonNullable<typeof functionProp>>>();
 });
+
+// @dts-jest:group WritableKeys
+it('WritableKeys', () => {
+  // @dts-jest:pass:snap
+  testType<WritableKeys<ReadWriteProps>>();
+});
+
+// @dts-jest:group ReadonlyKeys
+it('ReadonlyKeys', () => {
+  // @dts-jest:pass:snap
+  testType<ReadonlyKeys<ReadWriteProps>>();
+});

src/mapped-types.ts

@@ -221,3 +221,27 @@ export interface _DeepPartialArray<T> extends Array<DeepPartial<T>> { }
 export type _DeepPartialObject<T> = {
   [P in keyof T]?: DeepPartial<T[P]>;
 };
+
+type IfEquals<X, Y, A = X, B = never> =
+  (<T>() => T extends X ? 1 : 2) extends
+    (<T>() => T extends Y ? 1 : 2) ? A : B;
+
+/**
+ * WritableKeys
+ * @desc get union type of keys that are writable in object type `T`
+ * Credit: Matt McCutchen
+ * https://stackoverflow.com/questions/52443276/how-to-exclude-getter-only-properties-from-type-in-typescript
+ */
+export type WritableKeys<T extends object> = {
+  [P in keyof T]-?: IfEquals<{ [Q in P]: T[P] }, { -readonly [Q in P]: T[P] }, P>
+}[keyof T];
+
+/**
+ * ReadonlyKeys
+ * @desc get union type of keys that are readonly in object type `T`
+ * Credit: Matt McCutchen
+ * https://stackoverflow.com/questions/52443276/how-to-exclude-getter-only-properties-from-type-in-typescript
+ */
+export type ReadonlyKeys<T extends object> = {
+  [P in keyof T]-?: IfEquals<{ [Q in P]: T[P] }, { -readonly [Q in P]: T[P] }, never, P>
+}[keyof T];