Refine arguments for error interceptors

[xamgore]

Any is not really a good type for error handling. I believe they should be Request / Response objects.

[Alanscut]

A successful callback should return V（AxiosResponse）, but when there is an error, maybe it should return AxiosError insteadof V?

[xamgore]

Are you sure about AxiosError?

[Alanscut]

Actually I think there will be no problem even if I don’t change it.

[chinesedfan]

Sorry, we don't know which type the error is. Because it can be thrown from anywhere.

[xamgore]

@chinesedfan can we improve the type annotation by writing AxiosError | any? IDE would improve their intellisense significantly.

[chinesedfan]

@xamgore You can use a type assertion(#2013 (comment)) or contribute a type guard(#1863). In fact, I like type guard better.

[xamgore]

Type assertion requires a hidden knowledge of AxiosError type existence. Same with a type guard — you have to read about it in docs or the source code before usage.

On the other hand if onRejected accepts a union, people will be able to use optional chaining, and IDE will enumerate its inner fields:

export interface AxiosInterceptorManager<V> {
-  use(onFulfilled?: (value: V) => V | Promise<V>, onRejected?: (error: V) => any): number;
+  use(onFulfilled?: (value: V) => V | Promise<V>, onRejected?: (error: AxiosError | any) => any): number;
  eject(id: number): void;
}

[chinesedfan]

@xamgore AxiosError | any looks making sense, though it seems to be weird at first glance. Can you find some documents or example links about it? Then we can persuade more people.

[xamgore]

Typescript documentation has a similar case:

They define two interfaces, possibly intersecting:

interface Bird {
    fly();
    layEggs();
}

interface Fish {
    swim();
    layEggs();
}

function getSmallPet(): Fish | Bird {
    // ...
}

let pet = getSmallPet();
pet.layEggs(); // okay
pet.swim();    // errors

pet?.layEggs(); // okay
pet?.swim(); // okay, though swim() wasn't called

Then they provide a user type-guard:

function isFish(pet: Fish | Bird): pet is Fish {
    return (pet as Fish).swim !== undefined;
}

if (isFish(pet)) pet.swim() // ok

In our case Fish | Bird === AxiosError | any.

[chinesedfan]

I know that official example. And just thought any as a bit special. Thanks for your work and I will do some further investigating. Also waiting for others' opinions in #2729.

[ahuglajbclajep]

Using asserts x is T added in TypeScript 3.7, you might be able to write as follows.

function assertIsAxiosError<T>(err: any): asserts err is AxiosError<T> {
  if (err.isAxiosError) {
    throw new AssertionError("Not an AxiosError!");
  }
}

// in onRejected function
// type Data = { x: number };
try {
  assertIsAxiosError<Data>(err);
  console.log(err.response?.data.x); // err: AxiosError<Data>
} catch {
  console.log(err); // err: any
}

I would be happy if this information is helpful for you.