Steps for TS to infer types on interfaces

Incorporated in my React application is an object that I devised. Within this object, there is the following definition for Props:

type Props = { message: MessageTypes | MessageImgTypes; showTimeStamp: boolean; }

If we assume that MessageTypes consists of the structure shown:

interface MessageTypes { text: string; };

And MessageImgTypes has the following structure:

interface MessageImgTypes { text?: string; textImg : string; };

Upon attempting to access the textImg property of MessageImgTypes, an error arises indicating:

"Property 'textImg' does not exist on type 'MessageTypes | MessageImgTypes'." Despite being a union, the aforementioned property should indeed exist within the MessageImgType declaration. I contemplated using Narrowing to create two branches, but found that typeof and instanceof are incompatible with these interfaces.

typescript

Answer №1

When we talk about the union, it means that message can be either of type MessageTypes or MessageImgTypes. Since there is no 'textImg' property in the MessageTypes, this results in an error being generated.

There are various strategies we can employ to address this issue.

  1. Utilize the in operator: By using the in operator, we can verify if a particular key exists within the object, aiding the compiler in narrowing down the type:
if('textImg' in message) {
   message.textImg // no error
}
  1. Discriminated unions In discriminated unions, each member shares a common property with a unique value. Checking the value of that field helps narrow down the type, although it necessitates modifications in the interface structure:
interface MessageTypes { text: string; type: 'one' };

interface MessageImgTypes { text?: string; textImg : string; type: 'two' };

if(message.type === 'two') {
   message.textImg // no error
}
  1. Implement a custom type guard:
const isMessageImgTypes = (arg: MessageTypes | MessageImgTypes): arg is MessageImgTypes => {
   return 'textImg' in arg;
}

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