"What is the best way to determine the data type of an object retrieved from an API in TypeScript

Question

"What is the best way to determine the data type of an object retrieved from an API in TypeScript

Hey there, I'm currently developing a web application using Angular 2 and I'm focusing on implementing an exception handling mechanism. To achieve this, I've created a model that mirrors the object structure I will receive from the server (ERROR DTO).

Below is the code snippet for my error handler:

protected handleError(error, continuation: () => Observable<any>) {
        if (error.status == 401) {
            this.appContextService.redirectForLogin();
        }
        else if (typeof error == ErrorDTO) {
            return Observable.throw(error);
        }
        else {

            return Observable.throw(error);
        }
    }

It's important to note that APIs may throw objects other than my designated ErrorDTO. I need to account for this scenario in my error handling. The issue lies with the second 'else' statement where I check if typeof error == ErrorDTO, it doesn't seem to be working. I want to verify if the error DTO being received matches the type of ErrorDTO. The current implementation throws an error stating "Operator == cannot be applied to types string and type of error DTO." Could someone provide guidance on how to rectify this? Thank you.

typescript exception

Answer 1

Answer №1

It seems like your current approach may not be effective.

To clarify, when you use typeof error, it will almost always return 'object', unless the error is specifically undefined, in which case it will return 'undefined'.

You might want to consider using instanceof instead, as it functions more in line with what you are aiming for. For example,

(new Number(23)) instanceof Number

would return true.

However, if your ErrorDTO is an interface, the type information may be lost during transpilation (as interfaces cannot be used with

instanceof</code). Even if you were to declare it as a class, if the <code>error

object comes from JSON parsing, it will simply be treated as a plain object. This is because instanceof relies on prototypes to determine object instances, and all JSON-parsed objects have Object.prototype as their prototype.

One suggestion could be to handle errors differently. For instance, you could implement an errorCode property on your backend's error responses, and then create an interface in your client code:

interface BaseError {
    errorCode: number
    status: number
    // other properties...
}

Subsequently, you can have specific error interfaces extend BaseError. In this scenario, your handleError method would expect an object of type BaseError:

protected handleError(error: BaseError, continuation: () => Observable<any>) {
    if (error.status === 401) {
        // handle unauthorized error
    } else switch (error.errorCode) {
          case XXX:
             // handle specific error code XXX
             break;
          case YYY:
             // handle specific error code YYY
             break;
    }
}

I hope these suggestions prove helpful in improving your error handling process.

Answer 2

It seems like your current approach may not be effective.

To clarify, when you use typeof error, it will almost always return 'object', unless the error is specifically undefined, in which case it will return 'undefined'.

You might want to consider using instanceof instead, as it functions more in line with what you are aiming for. For example,

(new Number(23)) instanceof Number

would return true.

However, if your ErrorDTO is an interface, the type information may be lost during transpilation (as interfaces cannot be used with

instanceof</code). Even if you were to declare it as a class, if the <code>error

object comes from JSON parsing, it will simply be treated as a plain object. This is because instanceof relies on prototypes to determine object instances, and all JSON-parsed objects have Object.prototype as their prototype.

One suggestion could be to handle errors differently. For instance, you could implement an errorCode property on your backend's error responses, and then create an interface in your client code:

interface BaseError {
    errorCode: number
    status: number
    // other properties...
}

Subsequently, you can have specific error interfaces extend BaseError. In this scenario, your handleError method would expect an object of type BaseError:

protected handleError(error: BaseError, continuation: () => Observable<any>) {
    if (error.status === 401) {
        // handle unauthorized error
    } else switch (error.errorCode) {
          case XXX:
             // handle specific error code XXX
             break;
          case YYY:
             // handle specific error code YYY
             break;
    }
}

I hope these suggestions prove helpful in improving your error handling process.

"What is the best way to determine the data type of an object retrieved from an API in TypeScript

Answer №1

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