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The keyof operator takes an object type and produces a string or numeric literal union of its keys: type Point = { x: number; y: number }; type P = keyof Point; type P = keyof Point. What does 'They're at four. HTTP 420 error suddenly affecting all operations, What are the arguments for/against anonymous authorship of the Gospels. function pluck T, K extends keyof T > . Most instances of this error represent potential errors in the relevant code. TypeScript introduced a new type never, which indicates the values that will never occur. Material for junior catch-up. Below is a simplified example of my code which suffers from undefined being in the list of keys. The solution to resolve this issue correctly is to help TypeScript to infer the type. syosset high school club list; type 'string' is not assignable to type 'never' keyof. To learn more, see our tips on writing great answers. type any is not assignable to type never angular. You need to type the state, or else ts does not know what the attributes array is: const [tableAttributes, setTableAttributes] = useState<{attributes: string[]}>({ attributes: [], }) Here we tell ts, that tableAttributes is an object with attributes as a string array. type 'string' is not assignable to type 'never' keyof. Argument of type 'string' is not assignable to parameter of type 'keyof Condition". Type 'string' cannot be used to index type 'T'. argument of type string is not assignable to never; type 'string' is not assignable to type 'never json; type string is not assignable to type never, create slice; type 'string' is not assignable to type 'never'.ts(2322) jasmine; argument of type any is not assignable to type never in object; type 'string' is not assignable to type 'never object Luckily, Alex Okrushko provided a better solution that doesn't impact the consumer. Canadian of Polish descent travel to Poland with Canadian passport. type 'string' is not assignable to type 'never' keyof This isn't the sort of code you would want in your codebase however. The never type is used when you are sure that something is never going to occur. // Error: Type 'String' is not // assignable to type 'string'. what is madison club seating? Anyway, your solution works. Why? How to convert a string to number in TypeScript? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Could someone explains why in the example code below typescript gives the error message "const key: string Type 'any' is not assignable to type 'never' with boolean in interface in TS 3.5, https://devblogs.microsoft.com/typescript/announcing-typescript-3-5/, chore(typescript): update to typescript 3.7.x, passing tests with a workaround for typescript bug, Dynamic class instance property assignment is expecting, Some way to explicitly handle when the value of property. As of the 2.1 release, TypeScript lets you define types in terms of keys, so you can write a type like this: 4. const STATE = { Pending: 'Pending', Started: 'Started', Completed: 'Completed', }; type StateFromKeys = keyof typeof STATE; Then you can use that type any place you need to constrain the type of a variable, or a return, or whatever: Now it's inferred as an intersection in this context instead, and you get never here too because string & boolean is an empty intersection - there is no possible value you can give that will be safe for both cases.