Inspecting a union type with a TypeScript property validation

Question

Inspecting a union type with a TypeScript property validation

I have defined a union of two types in my code. Here are the definitions:

type Generic = {
  subtype: undefined,
  user: string,
  text: string
}

type Other = {
  subtype:'message',
  text: string
}

type Message = Generic | Other;

Within my code, I am using the following condition:

function getMessage(event: Message) {
  if (event.subtype === undefined) console.log(event.user); // This should work because only Generic has subtype === undefined and it has a user property
}

However, when running this code, TypeScript raises an error stating

Property user does not exist on type Message

. Shouldn't it work since it is within the if condition that checks for subtype? How can I resolve this issue, especially if these type definitions come from a library over which I have no control?

typescript

Answer 1

Answer №1

Whether the below code works correctly depends on the setting of strictNullChecks in the TypeScript configuration: if strictNullChecks is set to true, then it functions as intended.

 function getMessage(event: Message) {
       if (event.subtype === 'message'){
          console.log(event.user);
       }
 }

However, if strictNullChecks is set to false, you will need to use the as operator like this:

function getMessage(event: Message) {
  if (event.subtype === 'message'){
      console.log((event as Generic).user);
  }
}

Here is a working example.

Answer 2

Whether the below code works correctly depends on the setting of strictNullChecks in the TypeScript configuration: if strictNullChecks is set to true, then it functions as intended.

 function getMessage(event: Message) {
       if (event.subtype === 'message'){
          console.log(event.user);
       }
 }

However, if strictNullChecks is set to false, you will need to use the as operator like this:

function getMessage(event: Message) {
  if (event.subtype === 'message'){
      console.log((event as Generic).user);
  }
}

Here is a working example.

Inspecting a union type with a TypeScript property validation

Answer №1

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