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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Mapped array/tuple types not always working as expected","articleBody":"# Bug Report\r\n\r\n### 🔎 Search Terms\r\n\r\nMapped array types\r\n\r\n### 🕗 Version \u0026 Regression Information\r\n\r\nI noticed now in TS 4.1.2.\r\n\r\n### ⏯ Code and Playground Links\r\n\r\nThere are two problems.\r\n\r\nThe following has an error in the mapped type despite that I did it just like in https://github.com/microsoft/TypeScript/pull/26063 (as far as I know):\r\n\r\n```ts\r\ntype Constructor\u003cT = object, A extends any[] = any[], Static = {}\u003e = (new (...a: A) =\u003e T) \u0026 Static\r\n\r\ntype ElementTypeArrayToInstArray\u003cT extends Constructor[]\u003e = {\r\n\t[K in keyof T]: InstanceType\u003cT[K]\u003e // \u003c----------- ERROR\r\n}\r\ntype ArrayValues\u003cT extends any[]\u003e = T[keyof T]\r\ntype ElementTypes\u003cT extends Constructor[]\u003e = ArrayValues\u003cElementTypeArrayToInstArray\u003cT\u003e\u003e\r\n\r\ntype test = ElementTypes\u003c[typeof HTMLAnchorElement, typeof HTMLDivElement]\u003e\r\n```\r\n\r\n[Playground link with relevant code](https://www.typescriptlang.org/play?#code/C4TwDgpgBAwg9gOwM7AE4FcDGw6oDwAqUAvFHAEYBWE2ANFAIJQQAewECAJklAIYIgA2gF0SfASPoBlYL2ABLTGIDeAXwB8YgBQIIAdyhaAdCd4AuRgEoSmgtYBkUGXMUAoV6EhQAogBsIALYcwATgEAyoqLwgBHAAksjAEVEghMxsHNywiCgY2LgimqTKrgCQggDSUPIIUADWECBwAGZQBMIWCSj8mBChkISVwpoA9CNQeAC00zOzkz4ASgsA8guuqh5hjJHRAGq8vugQSGms7Fw8-ELDYgSCDU2t7ZtefoHB-cenGRfZiXk4VCFMTJPYHI4nN5BBAhMKgmLxRLwwjqdTuTzQdgoMRQj5hE6CDEtKAACQIAFkADIMBCYAAWuFxMPoRNaZKpABF5AA3JnAYauIA)\r\n\r\nBut notice that when you hover on `test`, it is a union of all the values of the array including values from non-numeric keys, which is unlike the examples in #26063.\r\n\r\nThere seems to some sort of special casing happening.\r\n\r\nI had to specify that I want `number` keys only in order(unlike #26063) to get rid of the error:\r\n\r\n```js\r\ntype Constructor\u003cT = object, A extends any[] = any[], Static = {}\u003e = (new (...a: A) =\u003e T) \u0026 Static\r\n\r\ntype ElementTypeArrayToInstArray\u003cT extends Constructor[]\u003e = {\r\n\t[K in keyof T]: InstanceType\u003cT[number \u0026 K]\u003e // \u003c----------- GOOD\r\n}\r\ntype ArrayValues\u003cT extends any[]\u003e = T[keyof T]\r\ntype ElementTypes\u003cT extends Constructor[]\u003e = ArrayValues\u003cElementTypeArrayToInstArray\u003cT\u003e\u003e\r\n\r\ntype test = ElementTypes\u003c[typeof HTMLAnchorElement, typeof HTMLDivElement]\u003e\r\n```\r\n\r\n[Playground link with relevant code](https://www.typescriptlang.org/play?#code/C4TwDgpgBAwg9gOwM7AE4FcDGw6oDwAqUAvFHAEYBWE2ANFAIJQQAewECAJklAIYIgA2gF0SfASPoBlYL2ABLTGIDeAXwB8YgBQIIAdyhaAdCd4AuRgEoSmgtYBkUGXMUAoV6EhQAogBsIALYcwATgEAyoqLwgBHAAksjAEVEghMxsHNywiCgY2LgimqTKrgCQggDSUPIIUADWECBwAGZQBMIWCSj8mBChkISCCOgB5BCoUI4VwpoA9LNQeAC0K6trS1AA4gDy2wAirqoeYYyR0QBqvL7oEEhprOxcPPxCM2IEgg1Nre3HXn6BYL9W73DJPbKJPI4VCFMTJC5XG53AFBBAhMLwmLxRKYwjqdTuTzQdgoMQooFhO6CIktKAACQIAFkADIMBCYAAWuHJaPoNNaDJZe3kADcecAZq4gA)\r\n\r\n\r\nBut notice that `test` still contains all the values of all keys, not just array values.\r\n\r\nFinally, I had to specify numeric keys for `ArrayValues`:\r\n\r\n```ts\r\ntype Constructor\u003cT = object, A extends any[] = any[], Static = {}\u003e = (new (...a: A) =\u003e T) \u0026 Static\r\n\r\ntype ElementTypeArrayToInstArray\u003cT extends Constructor[]\u003e = {\r\n\t[K in keyof T]: InstanceType\u003cT[number \u0026 K]\u003e\r\n}\r\ntype ArrayValues\u003cT extends any[]\u003e = T[number \u0026 keyof T] // \u003c-------- number\r\ntype ElementTypes\u003cT extends Constructor[]\u003e = ArrayValues\u003cElementTypeArrayToInstArray\u003cT\u003e\u003e\r\n\r\ntype test = ElementTypes\u003c[typeof HTMLAnchorElement, typeof HTMLDivElement]\u003e\r\n```\r\n\r\n[Playground link with relevant code](https://www.typescriptlang.org/play?#code/C4TwDgpgBAwg9gOwM7AE4FcDGw6oDwAqUAvFHAEYBWE2ANFAIJQQAewECAJklAIYIgA2gF0SfASPoBlYL2ABLTGIDeAXwB8YgBQIIAdyhaAdCd4AuRgEoSmgtYBkUGXMUAoV6EhQAogBsIALYcwATgEAyoqLwgBHAAksjAEVEghMxsHNywiCgY2LgimqTKrgCQggDSUPIIUADWECBwAGZQBMIWCSj8mBChkISCCOgB5BCoUI4Vwuquqh5hjJHRAGq8vugQSGms7Fw8-EIzYgRDI2MTjg1Nre1QAPT3UHgAtG-vb1DDo+MLXn6BYL9LY7DL7bKJPI4VCFMTJVbrTbbAFBBAhMLwmLxRKYwjqWZ-aDsFBiFFAsLbQSeCAtKAACQIAFkADIMBCYAAWuDJaPo1NpDJZABF5AA3HnAGauIA)\r\n\r\nBut notice if I use `Promise` instead of `InstanceType`, the first issues goes away, although `number` is still needed in `ArrayValues`:\r\n\r\n```ts\r\ntype Constructor\u003cT = object, A extends any[] = any[], Static = {}\u003e = (new (...a: A) =\u003e T) \u0026 Static\r\n\r\ntype ElementTypeArrayToInstArray\u003cT extends Constructor[]\u003e = {\r\n\t[K in keyof T]: Promise\u003cT[K]\u003e // \u003c-------- no number\r\n}\r\ntype ArrayValues\u003cT extends any[]\u003e = T[number \u0026 keyof T] // \u003c-------- number still needed\r\ntype ElementTypes\u003cT extends Constructor[]\u003e = ArrayValues\u003cElementTypeArrayToInstArray\u003cT\u003e\u003e\r\n\r\ntype test = ElementTypes\u003c[typeof HTMLAnchorElement, typeof HTMLDivElement]\u003e\r\n```\r\n\r\n[Playground link with relevant code](https://www.typescriptlang.org/play?#code/C4TwDgpgBAwg9gOwM7AE4FcDGw6oDwAqUAvFHAEYBWE2ANFAIJQQAewECAJklAIYIgA2gF0SfASPoBlYL2ABLTGIDeAXwB8YgBQIIAdyhaAdCd4AuRgEoSmgtYBkUGXMUAoV6EhQAogBsIALYcwATgEAyoqLwgBHAAksjAEVEghMxsHNywiCgY2LgimqTKrgCQggDSUPIIUADWECBwAGZQBMIWAAqocAHySBCElcKaAPSjUHgAtDOzM1AIcAvoAeQQqK6qHmGMkdEAary+6BBIaazsXDz8QiNiBIIIK2uoUI4NTa3tUOOTc3PLVbrKAoeS+XwLCAQTjQ7ZePyBYKhSBnIgXTI8eCJPI4VCFMTJA5HE5nBFBBAhMKEmLxRLUwjqdTuTzQdgoMRkpFhM6CFktKAACQIAFkADIMBCYAAWuE5FPofNaQrFABF5AA3OXAEauIA)\r\n\r\n\r\n\r\n### 🙁 Actual behavior\r\n\r\nDoes not work like #26063\r\n\r\n### 🙂 Expected behavior\r\n\r\nWorks like #26063\r\n\r\n---\r\n\r\nThere seems to be some sort of special casing going on.","author":{"url":"https://github.com/trusktr","@type":"Person","name":"trusktr"},"datePublished":"2021-03-07T21:58:15.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":5},"url":"https://github.com/43129/TypeScript/issues/43129"}