Theo's Typesafe Cult•3y ago

Conditional rendering on TS type

Hey everyone, I have a timeline that has multiple types coming over the wire in a list, and I need to render different ones depending on the type that it is. I seem to be getting an error on this, but not entirely sure how to correct. I tried using a comparison on typeof, but because that's comparing strings, it doesn't recognise that it's actually acting to guard the type. the TimelineUpdate type just requires a WrittenUpdate and as it's type, and the WrittenUpdate type is imported from @prisma/client at the top of this file Thanks 🙏

8 Replies

cje•3y ago

Types don’t exist at runtime

zenithOP•3y ago

Ok, interesting. I'll have to rethink this.

Vincent Udén•3y ago

Generally I'd say that using the type of a variable for control flow in the program is bad code smell. An options is having a broader update type with an enum on it which differentiates between the types of updates Or if the types are full blown classes, just define a common interface for them

Unknown User•3y ago

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Vincent Udén•3y ago

What? No. Pattern matching algebraic types is of course a staple of func. prog. So is overloading a function for different types. Using instance of isn't comparable at all imo

Unknown User•3y ago

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Vincent Udén•3y ago

No worries!

oldben•3y ago

if you have a union type then the 'in' operator is your friend for doing this:

interface Square { corners: number }
interface Circle { radius: number }
type Shape = Square | Circle
const whichShape = (shape: Shape) => {
  if ('radius' in shape) {
    // ... handle circle
  }
}

interface Square { corners: number }
interface Circle { radius: number }
type Shape = Square | Circle
const whichShape = (shape: Shape) => {
  if ('radius' in shape) {
    // ... handle circle
  }
}

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Conditional rendering on TS type