Anyways. If I have discriminated unions in TypeScript, how can I

Question

Anyways. If I have discriminated unions in TypeScript, how can I

make them narrow down to a single type using function maps?

Example:

type States = {
name: 'a',
someField: string
} | {
name: 'b',
otherField: boolean
}

const stateChange = (state: States) => {
const stateMap = {
a: () => state.someField,
b: () => state.otherField,
}
return stateMap[state.name]()
}

In the stateMap, TS doesn't know anymore that it was called with stateMap[state.name](). But with if and switch it works. I guess my fancy function maps do not work in this case. Otherwise I have to explicitly give the type for the function, maybe also not so bad.
Anyways, works perfectly with if and that is already great.

0

15.12.2020

3 ответов

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Misan Автор вопроса

Thomas
You should be able to cast it more strictly if you...

Makes sense. I just tried to put unions to a test after reading this article: https://blog.logrocket.com/put-the-typescript-enums-and-booleans-away/

0

15.12.2020

Thomas

https://www.typescriptlang.org/play?#code/C4TwDgpgBAysCGwIGcoF4oG8CwAoKBUAdvALYQBcUA5PNQDR6FTID25AYgJYQA2AJlWTAATlyIBzPAF8oAHyxNCJclWoAjBkoKtgACwgjufQVHWtWvCPCIy8eAMasiwlgiQBhPTYnQMACmFESlh3FABKdAA+RXxCJxdgN2CAWXgwdFjmZngqQLCqOGDUADIsYjIQ2mooaUi0GKCkADo2Th4BRjjsszymkKKkUvKVKs1a+saw5t0DIw7+LuZpbSgRCGAAVxEiZKQ0sABtfubRgF184Kh4VCIIADdDcIBuO1wgA

0

15.12.2020

Thomas · Accepted Answer

Thomas

You should be able to cast it more strictly if you find yourself fighting TS inference

0

15.12.2020

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Anyways. If I have discriminated unions in TypeScript, how can I

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