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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Using 'Key in keyof ArrayType' always returns an Array regardles of the value manipulation applied","articleBody":"\u003c!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨\r\n\r\nHalf of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common \"Bugs\" That Aren't Bugs section!\r\n\r\nPlease help us by doing the following steps before logging an issue:\r\n  * Search: https://github.com/Microsoft/TypeScript/search?type=Issues\r\n  * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ\r\n\r\nPlease fill in the *entire* template below.\r\n--\u003e\r\n\r\n**TypeScript Version:**  4.0.0-beta (same behaviour in version 3.5.1 [v3.5.1-Play Ground](https://www.typescriptlang.org/play?strictPropertyInitialization=false\u0026useDefineForClassFields=true\u0026allowUnusedLabels=true\u0026noEmitHelpers=true\u0026noUnusedLocals=true\u0026noUnusedParameters=true\u0026esModuleInterop=false\u0026preserveConstEnums=true\u0026declaration=false\u0026target=99\u0026jsx=0\u0026ts=3.5.1#))\r\n\r\n\u003c!-- Search terms you tried before logging this (so others can find this issue more easily) --\u003e\r\n**Search Terms:**\r\n\"key in keyof Array\", \"keyof Array\"\r\n\r\n**Expected behavior:**\r\nusing ```key in ArrayType``` has the same behaviour as on any object *(returning an array type only if the relevant properties are unchanged)*\r\n\r\n**Actual behavior:**\r\nusing ```key in ArrayType``` performs actions on touple entries but the returned type allways includes all default Array properties\r\n\r\n***Extention note:***\r\nNot the point of this issue and i know there are other issues around this topic. it's just a note,\r\nbut how about possibly adding tupple functionality for ArrayLike-, Map- and Set-types? They have all the basic qualities of a tupple, and it would be awesome to use them in a similar fashion (especially inference of ```Map.prototype.get/set``` and ```[Symbol.iterator]``` inference of tupples)\r\n\r\n\u003c!-- Did you find other bugs that looked similar? --\u003e\r\n**Related Issues:**\r\n#39726 \r\n#34780\r\n[type-fest/issues/119](https://github.com/sindresorhus/type-fest/issues/119) *(external library, but about this this behaviour and includes adHoc fix)*\r\n\r\n**Code**\r\n\r\n```ts\r\ntype Arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]\r\ntype Arr2 = (0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9)[]\r\ntype WhoopMyArray_Faulty\u003cBase extends ArrayLike\u003cany\u003e, Condition\u003e = {  \r\n\t\t[Key in keyof Base]:\r\n\t\t\tBase[Key] extends Condition\r\n\t\t\t\t? 'Whoop it'\r\n\t\t\t\t: Base[Key] extends Base[number] ? Base[Key] : never\r\n\t}\r\n\r\n// action is performed on array elements but return type is still a proper array\r\ntype WhoopedArray = WhoopMyArray_Faulty\u003cArr, 7 | 5 | 2\u003e\r\ntype WhoopedArrayKeys = WhoopMyArray_Faulty\u003cArr, 7 | 5 | 2\u003e[keyof Arr]\r\n\r\n// related side note, but not the main pont\r\n// no action performed. in line with 1st example, expected: (0 | 1 | 'Whoop it' | 3 | 4 | 'Whoop it' | 6 | 'Whoop it' | 8 | 9)[]\r\ntype WhoopedArray2 = WhoopMyArray_Faulty\u003cArr2, 7 | 5 | 2\u003e\r\ntype WhoopedArray2Keys = WhoopMyArray_Faulty\u003cArr, 7 | 5 | 2\u003e[keyof Arr]\r\n```\r\ninfered types:\r\n\r\n```d.ts\r\ntype WhoopedArray = [0, 1, \"Whoop it\", 3, 4, \"Whoop it\", 6, \"Whoop it\", 8, 9]\r\n\r\ntype WhoopedArrayKeys = 0 | \"Whoop it\" | 1 | 3 | 4 | 6 | 8 | 9 | 10 | (() =\u003e string) | (() =\u003e string) \r\n  | (() =\u003e 0 | \"Whoop it\" | 1 | 3 | 4 | 6 | 8 | 9 | undefined) | ((...items: (0 | \"Whoop it\" | 1 | 3 | 4 | 6 | 8 | 9)[]) =\u003e number)\r\n  | {    ...;    } | ... 22 more ... | (() =\u003e IterableIterator\u003c...\u003e)\r\n\r\ntype WhoopedArray2 = (0 | 7 | 5 | 2 | 1 | 3 | 4 | 6 | 8 | 9)[]\r\n\r\ntype WhoopedArray2Keys = 0 | \"Whoop it\" | 1 | 3 | 4 | 6 | 8 | 9 | 10 | (() =\u003e string) | (() =\u003e string)\r\n  | (() =\u003e 0 | \"Whoop it\" | 1 | 3 | 4 | 6 | 8 | 9 | undefined) | ((...items: (0 | \"Whoop it\" | 1 | 3 | 4 | 6 | 8 | 9)[]) =\u003e number)\r\n| {     ...;    } | ... 22 more ... | (() =\u003e IterableIterator\u003c...\u003e)\r\n```\r\n\r\n\r\n\u003cdetails\u003e\u003csummary\u003e\u003cb\u003eOutput\u003c/b\u003e\u003c/summary\u003e\r\n\r\n```ts\r\n\"use strict\";\r\n\r\n```\r\n\r\n\r\n\u003c/details\u003e\r\n\r\n\r\n\u003cdetails\u003e\u003csummary\u003e\u003cb\u003eCompiler Options\u003c/b\u003e\u003c/summary\u003e\r\n\r\n```json\r\n{\r\n  \"compilerOptions\": {\r\n    \"noImplicitAny\": true,\r\n    \"strictNullChecks\": true,\r\n    \"strictFunctionTypes\": true,\r\n    \"strictBindCallApply\": true,\r\n    \"noImplicitThis\": true,\r\n    \"noImplicitReturns\": true,\r\n    \"useDefineForClassFields\": true,\r\n    \"alwaysStrict\": true,\r\n    \"allowUnusedLabels\": true,\r\n    \"noEmitHelpers\": true,\r\n    \"noUnusedLocals\": true,\r\n    \"noUnusedParameters\": true,\r\n    \"preserveConstEnums\": true,\r\n    \"experimentalDecorators\": true,\r\n    \"emitDecoratorMetadata\": true,\r\n    \"moduleResolution\": 2,\r\n    \"target\": \"Latest\",\r\n    \"module\": \"ESNext\"\r\n  }\r\n}\r\n```\r\n\r\n\r\n\u003c/details\u003e\r\n\r\n**Playground Link:** [Provided](https://www.typescriptlang.org/play?strictPropertyInitialization=false\u0026useDefineForClassFields=true\u0026allowUnusedLabels=true\u0026noEmitHelpers=true\u0026noUnusedLocals=true\u0026noUnusedParameters=true\u0026esModuleInterop=false\u0026preserveConstEnums=true\u0026declaration=false\u0026target=99\u0026jsx=0\u0026ts=4.0.0-beta#code/C4TwDgpgBAggTnKBeKcIEMAmB7AdgGxCgG0AGAGigEZKAmSgZkoBZKBWSgNkoHZKAOSgE4AugFgAUJNCQoAdQAW2bGACyIeHHQgA+gDF0AV3ygAPACF0AZ2gQAHsAi5MV2Am0AZAJYBrCKfRcEAA+SgBhPEwvYC88YOQoAG8oKEkASDTiAGkIIi9cKD8QbAAzKEsbEQAudIyKiGzckSh7R2dXCOdo2NxajIB+KAByRWUwKGihvrSq8usGnJBm1qcXOZtiXEMAWwAjCDhmwfWFpqhZ3AgANwP0gF9JSQB6J6h0AGMYvAnXSDgS7BwbYQTBQb7odxECD4CDA3DAVy7QzACa4d74QyYCCudD4fBQK64wzYsFlTTaAB0YDg2GAtPAEGkDPkShUIPJIEWrhQoxU6g5+iMJhApk0vCgAB8oGxJVBaMFiEVSm5DkA)\r\n      ","author":{"url":"https://github.com/KilianKilmister","@type":"Person","name":"KilianKilmister"},"datePublished":"2020-08-15T16:11:42.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/40069/TypeScript/issues/40069"}