Object is of type 'unknown' typescript generics

Question

I have a simple function that takes a function as it's argument and returns a new function. I get Object is of type 'unknown' when calling the returned function

const makeFunction = <T>(callback: (someParam: number, options: T) => any) => {

  return (options: T) => {
    const param = 4;

    return callback(param, options)  
  }  
}

Above code is okay for typescript but when I am calling function I get complaint

makeFunction((param, options) => {
  const a = options.optionsValue //(parameter) options: unknown,  Object is of type 'unknown'
})({optionsValue: 'some value'})

Answer 1

We need to think how TS can infer the type from this definition. TS can understand type from two places:

• explicit generic type set
• type of second argument of the function

In your use case you don't provide type in any of those places and this is the reason you get unknown, because how TS can know what argument type you need. In order to give TS possibility to understand the type you can do or:

Set explicitly generic by:

makeFunction<YourType>((param, options) => {...))

Set type in callback function by for example defining one beforehand:

const f = (a: number, b: {a: string}) => b // here types are set
makeFunction(f)({a: 'some value'}) // makeFunction is able to infer the types by f

You can also do this inline by saying ((param: number, options: MyType))

Answer after comment if options can be dynamic

I believe you want below behavior:

const makeFunction = <F extends (someParam: number, options: any) => any>(callback: F) => {

  return (options: Parameters<F>[1]) => {
    const param = 4;

    return callback(param, options)  
  }  
}
const f = (a: number, b: {a: string}) => b
makeFunction(f)({ a: 'a' })
const g = (a: number, b: {b: number}) => b
makeFunction(g)({b: 1})

We say few things:

• F is now function which extends from binary function, and we directly infer its type
• Parameters<F>[1] is second argument type of given function F type

Answer 2

Since this is the first result you get when you google Object is of type 'unknown', I want to post my case. It might help future readers. This is not the answer to OP's question.

I got this error in the catch block. After debugging for a while I came to know that starting typescript v4.0, catch clause variables have type unknown instead of any.

And according to docs:

unknown is safer than any because it reminds us that we need to perform some sort of type-checks before operating on our values.

My code looked something like this before v4.0:

try {
  // try something exceptional here
} catch (error) {
  console.log(error.message);
}

And to fix this error, I had to put an additional if check on error variable.

try {
  // try something exceptional here
} catch (error) {
  let errorMessage = "Failed to do something exceptional";
  if (error instanceof Error) {
    errorMessage = error.message;
  }
  console.log(errorMessage);
}

Answer 3

updating my tsconfig.json with the following has worked for me:

"useUnknownInCatchVariables": false,

Update:

you need to put it like this under compilerOptions

"compilerOptions": {
    "useUnknownInCatchVariables": false
  }

Note: You need Typescript v4.4 or higher to be able to use this compiler option else you get a compiler error.

Answer 4

I got this issue in a try/catch bumping the Typescript to version 4.4 and found in the documentation the explanation for that.

In resume they added the flag useUnknownInCatchVariables that is true by default if strict. It basically changes the type of the error in a catch from any to unknown causing this issue.

So in order to bump to 4.4 you have some options:

try {
    ...
} catch(e) {
    console.log((e as Error).message)
}

or:

try {
    ...
} catch(e) {
    if (e instanceof Error) {
        console.log(e.message)
    }
}

or in your tsconfig.json you can explicitly set the flag to false:

{
    "compilerOptions": {
        "strict": true,
        "useUnknownInCatchVariables": false
    }
}

Answer 5

You can use casting: (err as Error)

I like the if statement solution, but this also works:

catch (error) {
  let msg = (error as Error).message;
}

Answer 6

Issue with TS 4.0 explained here: https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/#unknown-on-catch

All of the other answers didn't work for me but using the isAxiosError worked just fine:

   } catch (error) {
      if (axios.isAxiosError(error)) {
        const errResp = error.response;
        // Handle your error type safe here
      } else {
        // Handle the unknown
      }
    }

No Object is of type 'unknown'.ts(2571) error anymore.

Answer 7

Create an interface for options or use any as type,

makeFunction((param, options: any) => {

  const a = options.optionsValue;

  })({optionsValue: 'some value'});

Typescript takes options type as {} which causes issue during compilation.

Answer 8

Add a type to the calling function like this:

function(): Observable<any>

to avoid it from returnig unknown.