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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Assertions do not narrow types when used as operand of the comma operator.","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\u003c!--\r\nPlease try to reproduce the issue with the latest published version. It may have already been fixed.\r\nFor npm: `typescript@next`\r\nThis is also the 'Nightly' version in the playground: http://www.typescriptlang.org/play/?ts=Nightly\r\n--\u003e\r\n**TypeScript Version:**  4.1.0-dev.20201026\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:** assert comma operator\r\n\r\n**Code**\r\n\r\n`foo1` and `foo2` should be equivalent but the assertion in `foo2` does not narrow they typing of `param`.\r\n\r\n```ts\r\ndeclare function assert(value: any): asserts value;\r\n\r\nfunction foo1(param: number | null | undefined): number | null { \r\n  const val = param !== undefined;\r\n\r\n  if (val) {\r\n    assert(param !== undefined);\r\n    return param\r\n  } else {\r\n    return null;\r\n  }\r\n}\r\n\r\nfunction foo2(param: number | null | undefined): number | null { \r\n  const val = param !== undefined;\r\n  return val ? (assert(param !== undefined), param) : null;\r\n                                          // ^^^^^ Still typed as number | null | undefined\r\n}\r\n```\r\n\r\n**Expected behavior:**\r\n\r\nNo errors.\r\n\r\n**Actual behavior:**\r\n\r\n```\r\nType 'number | null | undefined' is not assignable to type 'number | null'.\r\n  Type 'undefined' is not assignable to type 'number | null'.(2322)\r\n```\r\n\r\n**Playground Link:** https://www.typescriptlang.org/play?ts=4.1.0-dev.20201026#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXygGdCQYMAKANygmRAC4DUBPASkaJLMPmtpADcAWABQolOmx4kOHAEZyAB1hQAto1TJVAI1LwAPvE0QIB+GlCIsqEMHZGtumGeOmA3vFHx4YPIQy8NPAAvPDKMGrwAITBoRYgVjbAwmIi3liI8FQ0rPBuXt4ExKQU4ZExcaiW1rasKYXwcBjIMPhlqgUAvvAgECR5Bd5NLfiu9fCdopOpEpi4+IiyAExKKuoOOnqGrmbxibUajlsOJnmeaT5+AXwhYWvRseZVCTXJBcOtgaYA-FmcJasIqoHpVqklWAAaO5A3KHEzjBqIpHIlGo+AAenR8AAerjcfAAMrYU4YZiKWxFDZOFzIU6GPavKZAA\r\n\r\n**Related Issues:** \u003c!-- Did you find other bugs that looked similar? --\u003e\r\n","author":{"url":"https://github.com/RebeccaStevens","@type":"Person","name":"RebeccaStevens"},"datePublished":"2020-10-27T02:27:26.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/41264/TypeScript/issues/41264"}