How to check for the property type of an `unknown` value?

Question

I have the following function, which could receive an unknown value:

function formatReason(detail: unknown): string {
    if (detail
        && detail instanceof Object
        && detail.constructor.name === 'Object'
        && detail.hasOwnProperty('description')
        && typeof detail['description'] === 'number'
    ) {
        const output = detail['description'];
        return output;
    }

    return '';
}

The detail parameter could be any value. If it's an object with a description property of type string, the function should return that property value, otherwise empty string.

First, do you recommend using any or unknown for the detail parameter?

Second, no matter what I do, the type for output ends up to be any. How can I make sure it is string?

Answer 1

Edit: got corrected by Neon. The typeguard was not sufficient enough. I updated the example to explicitly assert an unknown value instead of an implicit any.

---

I'd recommend using unknown as it is the type-safe variant of any, that being said you probably want to use a type guard to assert the unknown value. This has the effect that the description property you're looking for is actually being asserted as a string and not as an any.

The typeguard (see the playground to see what IDescription is):

public hasDescription(obj: unknown): obj is IDescription {     return (obj as IDescription).description !== undefined         && typeof (obj as IDescription).description === "string"; } 

Using it in the codebase will result in an if-statement with some benefits.

if (this.hasDescription(detail)) {     // In this if-block the TypeScript compiler actually resolved the unknown type to the type described in the guard.     console.log(detail.description); } 

Here's a playground for you to see how that works (note how only 123 is being outputted to the console).

---

An example implementation for your specific problem:

function formatReason(detail: unknown): string {     if (this.hasDescription(detail) {         return detail.description;     }      return ''; }  

Answer 2

There isn't a great way to write this code until this suggestion is implemented. In the meantime, it's up to you whether you prefer any or unknown (with some casts if you are using noImplicitAny, as I'd generally recommend). I wouldn't worry about the type of the output local variable because you've declared the return type of the function as string anyway.