Interesting idea, I wonder if you could write a custom wrapper for axios for your project and use a callback that checks if the AxiosResponse passes isUser's validation. if yes, you have the response, if no, you can throw the response as an error.

Otherwise, I can pick this feature, and write it in axios.

Instead of passing the isUser function to axios config. I think it would be nice to have a type object. Something like:

{
    firstname: [{
        type: "string",
       required: true 
    }],
    "lastname": [{
            type: "string",
           required: true 
    }]
}

That way you can just pass in the type, and get rid of all sort of type checking needed.

I really like using a data type guard to define the return type of the request.
And using a type guard function could also allow custom validator to be used, like a schema validator or any custom library.

Nice proposal! 👍

Wonder if a static-types-only solution would be a potentially less complicated alternative.

Eg. supporting configuration of the TypeScript types of the expected request and response for each request path (and checking the type of the request type) would yield most of the benefits without the downside having to run JS code at runtime.

Also, no need to duplicate the configuration { validateData: isUser } being passed every time that you want to make a call to /users/${id} - it will already be configured.

type PathTypes = [
  {
    method: 'PUT',
    path: `/users/${string}`,
    requestBody: {
      name: string,
    },
    responseBody: {
      name: string,
      favoriteColor: string,
    },
  },
];

const instance = axios.create<PathTypes>({
  baseURL: 'https://example.com/api/',
});

const user = await axios.put(`/users/${id}`, {
  namez: 'new name', // ❌ Type error: Property namez does not exist on type
});

user.favoriteColor // ✅ Type: string
user.favoriteColorrr // ❌ Type error: Property favoriteColorrr does not exist on type

Oh wait, maybe generic parameters already exist on instances? Just reading in this closed PR here: #4603

Edit: oh hm, I guess not - seems like axios.create() still doesn't accept generic parameters:

Yeah, it should actually work much better than it currently does. The team is working on a full v2 with much better TS support, I have added this to the backlog so I will try prioritise it.

should i close the issue ?