Property ‘userID’ does not exist on type ‘SignedInAuthObject | SignedOutAuthObject’.

Property ‘userID’ does not exist on type ‘SignedInAuthObject | SignedOutAuthObject’

If you’re working with TypeScript and encounter the error “Property ‘userID’ does not exist on type ‘SignedInAuthObject | SignedOutAuthObject’”, don’t worry, you’re not alone. This error typically occurs when you’re trying to access a property that is not present in all possible types of an object.

There are a few different solutions to this problem, depending on your specific use case. Let’s explore each solution and provide code snippets for each:

Solution 1: Type Assertion

One way to resolve this error is by using a type assertion. Type assertions allow you to override the inferred type of an expression and tell TypeScript to treat it as a different type. In this case, you can assert that the object is of type ‘SignedInAuthObject’ before accessing the ‘userID’ property.

interface SignedInAuthObject {
  userID: string;
  // other properties
}

interface SignedOutAuthObject {
  // other properties
}

function getUserID(authObject: SignedInAuthObject | SignedOutAuthObject) {
  if ('userID' in authObject) {
    return (authObject as SignedInAuthObject).userID;
  }
  return null;
}

const authObject: SignedInAuthObject | SignedOutAuthObject = // your object here
const userID = getUserID(authObject);
console.log(userID);

In this example, the ‘getUserID’ function checks if the ‘userID’ property exists in the ‘authObject’ using the ‘in’ operator. If it does, it asserts that ‘authObject’ is of type ‘SignedInAuthObject’ and returns the ‘userID’. Otherwise, it returns null.

Solution 2: Type Guards

Another solution is to use type guards. Type guards are functions or expressions that return a boolean value and allow you to narrow down the type of an object within a conditional block.

interface SignedInAuthObject {
  userID: string;
  // other properties
}

interface SignedOutAuthObject {
  // other properties
}

function isSignedInAuthObject(
  authObject: SignedInAuthObject | SignedOutAuthObject
): authObject is SignedInAuthObject {
  return 'userID' in authObject;
}

function getUserID(authObject: SignedInAuthObject | SignedOutAuthObject) {
  if (isSignedInAuthObject(authObject)) {
    return authObject.userID;
  }
  return null;
}

const authObject: SignedInAuthObject | SignedOutAuthObject = // your object here
const userID = getUserID(authObject);
console.log(userID);

In this example, the ‘isSignedInAuthObject’ function checks if the ‘userID’ property exists in the ‘authObject’ using the ‘in’ operator and returns a boolean value. The ‘getUserID’ function then uses this type guard to narrow down the type of ‘authObject’ within the conditional block.

These are two possible solutions to the error “Property ‘userID’ does not exist on type ‘SignedInAuthObject | SignedOutAuthObject’”. Depending on your specific use case, you can choose the solution that best fits your needs.

We hope this article has helped you resolve this TypeScript error. If you have any further questions, feel free to leave a comment below.