Object is of type 'unknown'

#

Quick Explanation

Object is of type 'unknown'.

This error is probably occurring because you're trying to access a property on something marked as unknown before narrowing it. You should either narrow it, or use a type assertion.

This is extremely common in try/catch blocks:

ts
try {
  // some code
} catch (e) {
  console.log(e.message);
'e' is of type 'unknown'.18046'e' is of type 'unknown'.
}

This is happening because unknown is considered to be any possible type. It's different from any because you can't access any properties on it without narrowing it first. With any, you can access any property on it without narrowing it.

#

Solution 1: Narrow the type

The first option is to narrow the type. You can do this by checking if the property exists on the object:

ts
try {
  // some code
} catch (e) {
  if (
    typeof e === "object" &&
    e &&
    "message" in e &&
    typeof e.message === "string"
  ) {
    // message gets narrowed to string!
    console.log(e.message);
                    
(property) message: string
  }
}

We first check if e is an object. Then, we check if it's not null or undefined. Then, we check if it has a message property. Finally, we check if e.message is a string.

TypeScript is smart enough to follow along with this logic and narrow e.message to a string!

#

Solution 2: Narrow the type using instanceof

If you're dealing with a class, you can use instanceof to narrow the type. This works especially well with errors:

ts
try {
  // some code
} catch (e) {
  if (e instanceof Error) {
    // e is narrowed to Error!
    console.log(e.message);
                    
(property) Error.message: string
  }
}

#

Solution 3: Use a type predicate

You can wrap your narrowing logic in a custom type predicate:

ts
function hasMessage(x: unknown): x is { message: string } {
  return Boolean(
    typeof x === "object" &&
      x &&
      "message" in x &&
      typeof x.message === "string"
  );
}
 
try {
  // some code
} catch (e) {
  if (hasMessage(e)) {
    // e is narrowed to { message: string }!
    console.log(e.message);
                    
(property) message: string
  }
}

This is useful if you want to reuse the same logic in multiple places.

#

Solution 4: Use a type assertion

If you're sure that the type is correct, you can use a type assertion to tell TypeScript that it's correct:

ts
try {
  // some code
} catch (e) {
  console.log((e as Error).message);
}

This is the least type-safe option, but it's also the easiest to use. Just be careful that you're not asserting the wrong type!