Generics 在 TypeScript 中起作用

Question

I was reading this article about how to use Axios with TypeScript and I had a question about the methods declaration, for example:

  get<T = any, R = AxiosResponse<T>>(url: string, config?: AxiosRequestConfig): Promise<R> {
    return this.http.get<T, R>(url, config);
  }

  delete<T = any, R = AxiosResponse<T>>(url: string, config?: AxiosRequestConfig): Promise<R> {
    return this.http.delete<T, R>(url, config);
  }

Now, what I miss to understand is the this syntax:

<T = any, R = AxiosResponse<T>>

Why is the T equal to any , isn't it the concept of Generics that T can be anything? Why do I need to specify it? Is it for readability? Also why is R equals AxiosResponse<T> ? Couldn't I just say that the return type of the function is Promise<AxiosResponse<T>> , is it also the sake of writing less code in the return type?

Answer 1

Those are defaults that are used when the type isn't specified and can't be inferred.

...isn't it the concept of Generics that T can be anything?

Not necessarily, generic type parameters can be constrained , but yes in this case.

Why do I need to specify it?

You don't need to (but you can), there's a default if you don't and it can't be inferred from the call site.

Also why is R equals AxiosResponse<T> ? Couldn't I just say that the return type of the function is Promise<AxiosResponse<T>> ...

It's simpler (and probably better for inference) to use if you specify the type of what you're expecting in the response, without the Promise wrapper on it.