#5181 - Mutable Keys (#5182)

Co-authored-by: Yugang Cao <34439652+Talljack@users.noreply.github.com>

questions/5181-hard-mutable-keys/README.md

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+Implement the advanced util type MutableKeys<T>, which picks all the mutable (not readonly) keys into a union.
+
+For example:
+
+```ts
+type Keys = MutableKeys<{ readonly foo: string; bar: number }>;
+// expected to be “bar”
+```

questions/5181-hard-mutable-keys/info.yml

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+difficulty: hard
+title: Mutable Keys
+tags: utils
+related: '2793'
+author:
+  github: Talljack
+  name: Yugang Cao
+

questions/5181-hard-mutable-keys/template.ts

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+type MutableKeys<T> = any

questions/5181-hard-mutable-keys/test-cases.ts

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+import { Equal, Expect } from '@type-challenges/utils'
+
+type cases = [
+    Expect<Equal<MutableKeys<{ a: number, readonly b: string }>, "a">>,
+    Expect<Equal<MutableKeys<{ a: undefined, readonly b: undefined }>, "a">>,
+    Expect<Equal<MutableKeys<{ a: undefined, readonly b?: undefined, c: string, d: null }>, "a" | "c" | "d">>,
+    Expect<Equal<MutableKeys<{}>, never>>
+]