feat: ➕ add MarkReadonly (#298)

* feat: ➕ add MarkReadonly * test: 🧪 MarkReadonly * docs: 📄 README * docs: 📄 changeset
ts-essentials · Jan 31, 2022 · e76a08a · e76a08a
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diff --git a/.changeset/seven-socks-beam.md b/.changeset/seven-socks-beam.md
@@ -0,0 +1,5 @@
+---
+"ts-essentials": minor
+---
+
+Add `MarkReadonly` which sets readonly for specific keys
diff --git a/README.md b/README.md
@@ -64,6 +64,7 @@ If you use any [functions](https://github.com/krzkaczor/ts-essentials/blob/maste
   - [MergeN](#MergeN)
   - [MarkRequired](#MarkRequired)
   - [MarkOptional](#MarkOptional)
+  - [MarkReadonly](#MarkReadonly)
   - [ReadonlyKeys](#ReadonlyKeys)
   - [WritableKeys](#WritableKeys)
   - [OptionalKeys](#OptionalKeys)
@@ -711,6 +712,30 @@ type UserWithoutPassword = MarkOptional<User, "password">;
 // }
 ```
 
+### MarkReadonly
+
+Useful when you want to make some properties readonly without creating a separate type.
+
+```typescript
+interface User {
+  id: number;
+  name: string;
+  email: string;
+  password: string;
+}
+
+type UserThatCannotChangeName = MarkReadonly<User, "name">;
+
+// Result:
+
+// {
+//   id: number;
+//   readonly name: string;
+//   email: string;
+//   password: string;
+// }
+```
+
 ### ReadonlyKeys
 
 Gets keys of an object which are readonly.

diff --git a/lib/types.ts b/lib/types.ts
@@ -394,6 +394,9 @@ export type MarkRequired<T, RK extends keyof T> = Omit<T, RK> & Required<Pick<T,
 /** Mark some properties as optional, leaving others unchanged */
 export type MarkOptional<T, K extends keyof T> = Omit<T, K> & Partial<Pick<T, K>>;
 
+/** Mark some properties as readonly, leaving others unchanged */
+export type MarkReadonly<T, K extends keyof T> = Omit<T, K> & Readonly<Pick<T, K>>;
+
 /** Convert union type to intersection #darkmagic */
 export type UnionToIntersection<U> = (U extends any ? (k: U) => void : never) extends (k: infer I) => void ? I : never;
 

diff --git a/test/mark-readonly.ts b/test/mark-readonly.ts
@@ -0,0 +1,34 @@
+import { AssertTrue as Assert, IsExact } from "conditional-type-checks";
+import { MarkReadonly, WritableKeys, ReadonlyKeys } from "../lib/types";
+
+function testMarkReadonly() {
+  type Example = {
+    readonly readonly1: Date;
+    readonly readonly2: RegExp;
+    required1: number;
+    required2: string;
+    optional1?: null;
+    optional2?: boolean;
+  };
+
+  type cases = [
+    Assert<IsExact<MarkReadonly<Example, never>, Example>>,
+    Assert<IsExact<MarkReadonly<Example, ReadonlyKeys<Example>>, Example>>,
+    Assert<IsExact<MarkReadonly<Example, WritableKeys<Example>>, Readonly<Example>>>,
+    Assert<
+      IsExact<
+        MarkReadonly<Example, "required1">,
+        {
+          readonly readonly1: Date;
+          readonly readonly2: RegExp;
+          readonly required1: number;
+          required2: string;
+          optional1?: null;
+          optional2?: boolean;
+        }
+      >
+    >,
+    // @ts-expect-error do NOT support union types
+    MarkReadonly<Example | { a: 1 }, "required1">,
+  ];
+}