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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Mapped tuples types iterates over all properties","articleBody":"**TypeScript Version:**  3.2.0-dev.20181019\r\n\r\n**Search Terms:** mapped tuples reify\r\n\r\n**Code**\r\n\r\n```ts\r\ntype Foo = ['a', 'b'];\r\ninterface Bar\r\n{\r\n\ta: string;\r\n\tb: number;\r\n}\r\n\r\ntype Baz = { [K in keyof Foo]: Bar[Foo[K]]; }; // Expected Baz to be [string, number]\r\n```\r\n\r\n**Expected behavior:** Baz should be [string, number]\r\n\r\n**Actual behavior:** Type '[\"a\", \"b\"][K]' cannot be used to index type 'Bar'.\r\n\r\n**Playground Link:** https://www.typescriptlang.org/play/index.html#src=type%20Foo%20%3D%20%5B'a'%2C%20'b'%5D%3B%0D%0Ainterface%20Bar%0D%0A%7B%0D%0A%09a%3A%20string%3B%0D%0A%09b%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Atype%20Baz%20%3D%20%7B%20%5BK%20in%20keyof%20Foo%5D%3A%20Bar%5BFoo%5BK%5D%5D%3B%20%7D%3B%20%2F%2F%20Expected%20Baz%20to%20be%20%5Bstring%2C%20number%5D%0D%0A%0D%0Atype%20WorkingBaz%20%3D%20%7B%20%5BK%20in%20Exclude%3Ckeyof%20Foo%2C%20keyof%20any%5B%5D%3E%5D%3A%20Foo%5BK%5D%20extends%20keyof%20Bar%20%3F%20Bar%5BFoo%5BK%5D%5D%20%3A%20never%3B%20%7D%20%26%20%7B%20length%3A%20Foo%5B'length'%5D%3B%20%7D%20%26%20any%5B%5D%3B\r\n\r\n**Related Issues:** https://github.com/Microsoft/TypeScript/issues/25947\r\n\r\n\r\nGiven the Mapped tuple types feature (#25947). I'd expect the code above to work cleanly.\r\n\r\nHowever, I still need to do:\r\n```ts\r\ntype WorkingBaz = { [K in Exclude\u003ckeyof Foo, keyof any[]\u003e]: Foo[K] extends keyof Bar ? Bar[Foo[K]] : never; } \u0026 { length: Foo['length']; } \u0026 any[];\r\n```\r\n\r\nTo have an equivalent type. As far as I understand, the \"K\" in a mapped type on a tuple should iterate only on numeric keys of this tuple. Therefore, Foo[K] should always be a valid key for Bar...","author":{"url":"https://github.com/lemoinem","@type":"Person","name":"lemoinem"},"datePublished":"2018-10-19T19:16:31.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":37},"url":"https://github.com/27995/TypeScript/issues/27995"}