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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Trying to flatten an array with unknown depth causes a type error","articleBody":"# Bug Report\r\n\r\nTrying to flatten an array with unknown depth causes a type error.\r\n\r\n### 🔎 Search Terms\r\n\r\n\u003c!--\r\n  What search terms did you use when trying to find an existing bug report?\r\n  List them here so people in the future can find this one more easily.\r\n--\u003e\r\n * `flat`\r\n * `Infinity`\r\n\r\n### 🕗 Version \u0026 Regression Information\r\n\r\n- Errors in version 4.2.4\r\n- This changed between versions 3.6.2 and 3.7.5 (but probably just because the type definitions aren't supported prior)\r\n\r\nThe PR https://github.com/microsoft/TypeScript/pull/32131 provides a definition for `FlatArray` that has this issue.\r\n\r\n### ⏯ Playground Link\r\n\r\n[Playground link with relevant code](https://www.typescriptlang.org/play?target=99\u0026ts=4.3.2\u0026filetype=ts#code/PTAEDMEsCcGcBdQBMCmUB2LQENSYSkqADaQKgDuk8AFjtAEbXTbQCeyKADrQFDxsuWAHIoCSADwAVAHygAvKAAUU0AB9Qo8dJkBKANoBdANy9eAYwD26cq2gAuTWPiEJ6AK4BbBimhzF+gCMADT6AEyhAMyGMaa8ICSW5tjExGzBoLRkoEyI4NiQxLAQltCgnlhKAG6+sJDWoAAsAHRhzY268cBdlNR02OigvtCloADkAETQKObu0BNjoNnT4L4o6OaEyDAz8Gmgo5DoSDvmexxHS-DFlhSDAkI46OiW8Njw9ejNFtbk07CODzeXxGBT0aDNcDEd5KACS6Aw1DYujiCTYlncoGSgyQlhwoAAju5IOYANaUUqk1gY444YrgSypW4An42RD-MKArw+aCgxRKOx0p5sIy6SHQ+BwhFHJEoswJBjuPKjWCWCqgabYVWDBq0LBSQQoADK5mgkB4oC40LYAHMRu5aVlirixOgxnkCsQMorEOjMdihp53BKsNQegwOO46ugbZkaFgGUyqDHOIiPrqaO8lsUUETIFUUutEPA8XqDphLSMqpBUERLmWEAMkKwiKQGCx2PYejR4PAuACQDa+u4GM0rJ5gJ4SSNVeB4MADUITWaeMAGMRLAxgEgABwoACc4AAbPuAKyBfcX7CRRoAdgYDECp9PDH3kR32FPAAYv0fwPu0GwB9b1vHdgDbcDIFHMQwi-C9mjsbA2GaJBmmuABiAAZMJAn4Q1QAAMQlABBaAWDYMIJFI6AMgAEW4WghgADxcY5iiBHl-FAABvXhQH40AJlxTAJkcajgj4gSphmOZRNAajmNYpBigAJRQbBhLSaikIkI5VjKeFMGgaiZEkgSBIAfkIkiyKQyjDN8cTQH0ABaEJQC-DJ3IiUBIgyRoMlPDIjwyW8Mh3DJ9y8zzQECdzAh8wI-NigLYqC2KQtisLYoi2KotAWDDH0eieBoQxTPM8yxLI3gAF9ioYugUBY9ZlNANzQCsoTrBQCZQEcaTZnmEx5TAZIozEfEHiwYZSkcSZpiGhZsw1NA1g2LYTkW84DjKI4tt2fZLmoG47kyfCBheN50y+PDHikZwiPeHQwSe+BtIoiQtBcSRZAyabLHAUB4TTNgZCAA)\r\n\r\n### 💻 Code\r\n\r\n```ts\r\n// first define a nested list with arbitrary depth\r\ntype Nested\u003cT\u003e = (T | Nested\u003cT\u003e)[];\r\n\r\nconst arr: Nested\u003cnumber\u003e = [1,[2,[3]]];\r\n\r\n// locally, this bit fails for me (version 4.2.4)\r\n//\r\n// with an error '\"recur\"' is referenced directly or indirectly in its own type annotation.\r\nconst res: number[] = arr.flat(Infinity);\r\n\r\n// you can do a quick workaround as follows:\r\nconst res2: number[] = (arr as any[]).flat(Infinity);\r\n\r\n// but for some reason on the TypeScript playground this doesn't fail, but you can emulate it\r\n// by using the following definition that is equivalent to the one provided in the standard library:\r\n// https://github.com/microsoft/TypeScript/blob/d8e9f6951919a347bb155b938a5006f9efabb778/lib/lib.es2019.array.d.ts#L21\r\ntype FlatArray2\u003cArr, Depth extends number\u003e = {\r\n    \"done\": Arr,\r\n    \"recur\": Arr extends ReadonlyArray\u003cinfer InnerArr\u003e\r\n        ? FlatArray2\u003cInnerArr, [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20][Depth]\u003e\r\n        : Arr\r\n}[Depth extends -1 ? \"done\" : \"recur\"];\r\n\r\n// causes a type error: '\"recur\"' is referenced directly or indirectly in its own type annotation.\r\ntype TestFlat\u003cT\u003e = FlatArray2\u003cNested\u003cT\u003e, typeof Infinity\u003e\r\n```\r\n\r\n### 🙁 Actual behavior\r\n\r\nThe code errors with `'\"recur\"' is referenced directly or indirectly in its own type annotation.`\r\n\r\n### 🙂 Expected behavior\r\n\r\nThe code to pass typechecking.\r\n","author":{"url":"https://github.com/GuiltyDolphin","@type":"Person","name":"GuiltyDolphin"},"datePublished":"2021-06-03T11:07:04.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/44408/TypeScript/issues/44408"}