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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Add type operators that can answer \"given function type T, what is the type of its return value when arguments P are passed in?\"","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.\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, especially the \"Common Feature Requests\" section: https://github.com/Microsoft/TypeScript/wiki/FAQ\r\n\r\n--\u003e\r\n\r\n## Search Terms\r\n\r\n\u003c!-- List of keywords you searched for before creating this issue. Write them down here so that others can find this suggestion more easily --\u003e\r\noverload infer function operator arguments parameters return\r\n\r\n## Suggestion\r\n\r\n```ts\r\n// Already possible\r\ntype ValueForKey\u003cT, K extends keyof T\u003e = T[K];\r\n\r\n// Proposal\r\ntype Callable = (...args: any[]) =\u003e any;\r\ntype ValueForArguments\u003cT extends Callable, A extends paramsof T\u003e = T(...A);\r\n```\r\n\r\n## Use Cases\r\n\r\nThis is a proposal that would help with overloaded function issues like #26591 and would help my [type testing library](https://humeris.org/espresso-shot) be able to support checks like `expectTypeOf(fn).toBeCallableWith(args)`.\r\n\r\nNote that though `ValueForKey` could be implemented as:\r\n```ts\r\ntype ValueForKey\u003cT, K extends keyof T\u003e = \r\n  T extends {[Key in K]: infer U} ? U : never;\r\n```\r\nThe following will only work for non-overloaded functions:\r\n```ts\r\ntype ValueForArguments\u003cT extends Callable, P extends Parameters\u003cT\u003e\u003e = \r\n  T extends (...args: P) =\u003e infer R ? R : never;\r\n```\r\nsince both the inference from `Parameters\u003cT\u003e` and the type definition will only consider one of the overloads.\r\n\r\nWhat I'm asking is not for inference to consider multiple overloads, which would be nice but might complexify the implementation of type inference, but direct support for `paramsof` and `T(A1, A2, ...A)` type syntax which behaves correctly with overloaded functions.\r\n\r\n## Examples\r\n```ts\r\ninterface OverloadedFunction {\r\n  (arg: number): number;\r\n  (arg: string): string;\r\n}\r\n\r\n// `number`\r\ntype Result1 = OverloadedFunction(number);\r\n\r\n// `number | string`\r\ntype Result2 = OverloadedFunction(number | string);\r\n\r\n// `[number] | [string]`\r\ntype Result3 = paramsof OverloadedFunction;\r\n\r\n// `number | string`\r\ntype Result4 = OverloadedFunction(...paramsof OverloadedFunction)\r\n```\r\n\r\n\u003c!--\r\nWhat do you want to use this for?\r\nWhat shortcomings exist with current approaches?\r\n--\u003e\r\n\r\n\r\n\r\n\u003c!-- Show how this would be used and what the behavior would be --\u003e\r\n\r\n## Also\r\nGiven the above, it would also make sense to have something like\r\n```ts\r\ntype Newable = new (...args: any[]) =\u003e any;\r\ntype ValueForArguments\u003cT extends Newable, A extends paramsof new T\u003e = new T(...A);\r\n```\r\nwhere `new T` takes the constructor type of `T` and turns it into a function type\r\n\r\n## Checklist\r\n\r\nMy suggestion meets these guidelines:\r\n\r\n* [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code\r\n* [x] This wouldn't change the runtime behavior of existing JavaScript code\r\n* [x] This could be implemented without emitting different JS based on the types of the expressions\r\n* [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)\r\n* [x] This feature would agree with the rest of [TypeScript's Design Goals](https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals).\r\n","author":{"url":"https://github.com/lazytype","@type":"Person","name":"lazytype"},"datePublished":"2020-08-21T11:46:03.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":8},"url":"https://github.com/40179/TypeScript/issues/40179"}