René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Add additional overloads to Array.prototype.concat","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. Please read the FAQ first, especially the \"Common Feature Requests\" section.\r\n\r\n--\u003e\r\n\r\n## Search Terms\r\n\r\nconcat never\r\n\r\n## Suggestion\r\n\r\nAdd the following signatures to the `Array` interface:\r\n\r\n```ts\r\ninterface Array\u003cT\u003e {\r\n    concat\u003cU\u003e(this: Array\u003cnever\u003e, ...items: ConcatArray\u003cU\u003e[]): U[];\r\n    concat\u003cU\u003e(this: Array\u003cnever\u003e, ...items: (U | ConcatArray\u003cU\u003e)[]): U[];\r\n}\r\n```\r\n\r\nFor my use case this is not needed in `ReadonlyArray`, but could be added for consistency.\r\n\r\n## Use Cases\r\n\r\nI often find myself in the need of using `Array.prototype.flatMap`, but since I target Node.js 6 it's not available. Therefore I try to replace it with `[].concat(someArray.map(mightReturnArray))`.\r\nBut then I get an error like `Type 'whatever' is not assignable to type 'never'.` (with `strictNullChecks` enabled).\r\n\r\n## Examples\r\n\r\n```ts\r\ninterface Array\u003cT\u003e {\r\n    /**\r\n     * Combines two or more arrays.\r\n     * @param items Additional items to add to the end of array1.\r\n     */\r\n    concat\u003cU\u003e(this: Array\u003cnever\u003e, ...items: ConcatArray\u003cU\u003e[]): U[];\r\n    /**\r\n     * Combines two or more arrays.\r\n     * @param items Additional items to add to the end of array1.\r\n     */\r\n    concat\u003cU\u003e(this: Array\u003cnever\u003e, ...items: (U | ConcatArray\u003cU\u003e)[]): U[];\r\n}\r\n\r\n// now OK\r\n[].concat(['a']);\r\n['a'].concat(['b']);\r\n\r\n// still an error as expected\r\n[1].concat(['a']);\r\n```\r\n\r\nhttp://www.typescriptlang.org/play/#src=interface%20Array%3CT%3E%20%7B%0A%20%20%20%20%2F**%0A%20%20%20%20%20*%20Combines%20two%20or%20more%20arrays.%0A%20%20%20%20%20*%20%40param%20items%20Additional%20items%20to%20add%20to%20the%20end%20of%20array1.%0A%20%20%20%20%20*%2F%0A%20%20%20%20concat%3CU%3E(this%3A%20Array%3Cnever%3E%2C%20...items%3A%20ConcatArray%3CU%3E%5B%5D)%3A%20U%5B%5D%3B%0A%20%20%20%20%2F**%0A%20%20%20%20%20*%20Combines%20two%20or%20more%20arrays.%0A%20%20%20%20%20*%20%40param%20items%20Additional%20items%20to%20add%20to%20the%20end%20of%20array1.%0A%20%20%20%20%20*%2F%0A%20%20%20%20concat%3CU%3E(this%3A%20Array%3Cnever%3E%2C%20...items%3A%20(U%20%7C%20ConcatArray%3CU%3E)%5B%5D)%3A%20U%5B%5D%3B%0A%7D%0A%0A%2F%2F%20now%20OK%0A%5B%5D.concat(%5B'a'%5D)%3B%0A%5B'a'%5D.concat(%5B'b'%5D)%3B%0A%0A%2F%2F%20still%20an%20error%20as%20expected%0A%5B1%5D.concat(%5B'a'%5D)%3B\r\nRemember to enable `strictNullChecks`\r\n\r\n## Checklist\r\n\r\nMy suggestion meets these guidelines:\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. new expression-level syntax)\r\n\r\n","author":{"url":"https://github.com/ajafff","@type":"Person","name":"ajafff"},"datePublished":"2018-09-08T12:54:03.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/26976/TypeScript/issues/26976"}