[solution]: en/medium-omitbytype.md

en/medium-omitbytype.md

@@ -0,0 +1,71 @@
+---
+id: 2852
+title: OmitByType
+lang: en
+level: medium
+tags: object
+---
+
+## Challenge
+
+From `T`, pick a set of properties whose type are not assignable to `U`. For
+example:
+
+```typescript
+type OmitBoolean = OmitByType<
+  {
+    name: string;
+    count: number;
+    isReadonly: boolean;
+    isEnable: boolean;
+  },
+  boolean
+>; // { name: string; count: number }
+```
+
+## Solution
+
+The example in this challenge already gives a hint that we need to work with
+mapped types. Having one object type, we need to create another one which will
+be a subset of the first one.
+
+We will start with the blank type that needs to be implemented:
+
+```typescript
+type OmitByType<T, U> = any;
+```
+
+The goal here is to make a subset, so let's start with a copy of the entire
+object. It is a classic implementation that uses mapped types:
+
+```typescript
+type OmitByType<T, U> = { [P in keyof T]: T[P] };
+```
+
+What is happening here is enumerating all the properties from type parameter `T`
+which are going into a new object type we created. That way we have a copy.
+
+Now, we need to filter some properties somehow. To do so, we can use remapping.
+We need to check if the type of the value matches the one that have been
+provided:
+
+```typescript
+type OmitByType<T, U> = {
+  [P in keyof T as T[P] extends U ? never : never]: T[P];
+};
+```
+
+If the value type matches the provided in type parameter `U`, we need to return
+`never` type. That way, the property will not be included in the new object.
+However, if the types are not matching, we need to return the property as is:
+
+```typescript
+type OmitByType<T, U> = { [P in keyof T as T[P] extends U ? never : P]: T[P] };
+```
+
+## References
+
+- [Generics](https://www.typescriptlang.org/docs/handbook/2/generics.html)
+- [Mapped Types](https://www.typescriptlang.org/docs/handbook/2/mapped-types.html)
+- [Key Remapping via as](https://www.typescriptlang.org/docs/handbook/2/mapped-types.html#key-remapping-via-as)
+- [Conditional Types](https://www.typescriptlang.org/docs/handbook/2/conditional-types.html)