Fixing an issue that type 'null' and 'undefined' is not assignable to validateStatus when typescript strict option is enabled

[cprayer]

In previous my PR(#2773), I had missed about two things.

1. Assign function type (one status number parameter, return boolean or null type) instead of function type(one status number parameter, return boolean type), null and undefined type
2. Doesn't care about undefined type

So I fix above things in this PR.

[chinesedfan]

Thanks for your update. And in order to avoid similar mistakes, can you update the test file with a new case? In addition, validateStatus? has included undefined I think.

[chinesedfan]

@cprayer Ping for updating.

[cprayer]

@chinesedfan
Thanks your review! I have missed about this PR.
As you said, I change the ValidStatus option to ((status: number) => boolean) | null and add some test case about null and undefined.

Before -> validateStatus?: ((status: number) => boolean | null)

tsc test/typescript/axios.ts --target es6 --moduleResolution node --strict
test/typescript/axios.ts:48:3 - error TS2322: Type 'null' is not assignable to type '((status: number) => boolean) | undefined'.

48   validateStatus: null
     ~~~~~~~~~~~~~~

  index.d.ts:65:3
    65   **validateStatus?: ((status: number) => boolean) | null**
         ~~~~~~~~~~~~~~
    The expected type comes from property 'validateStatus' which is declared here on type 'AxiosRequestConfig'

test/typescript/axios.ts:341:9 - error TS2345: Argument of type '(value: string) => void' is not assignable to parameter of type '(value: string | AxiosResponse<any>) => void | PromiseLike<void>'.
  Types of parameters 'value' and 'value' are incompatible.
    Type 'string | AxiosResponse<any>' is not assignable to type 'string'.
      Type 'AxiosResponse<any>' is not assignable to type 'string'.

341   .then((value: string) => {});
            ~~~~~~~~~~~~~~~~~~~~~

test/typescript/axios.ts:345:9 - error TS2345: Argument of type '(value: string) => void' is not assignable to parameter of type '(value: string | AxiosResponse<any>) => void | PromiseLike<void>'.
  Types of parameters 'value' and 'value' are incompatible.
    Type 'string | AxiosResponse<any>' is not assignable to type 'string'.
      Type 'AxiosResponse<any>' is not assignable to type 'string'.

345   .then((value: string) => {});
            ~~~~~~~~~~~~~~~~~~~~~

After - validateStatus?: ((status: number) => boolean) | null

tsc test/typescript/axios.ts --target es6 --moduleResolution node --strict
test/typescript/axios.ts:341:9 - error TS2345: Argument of type '(value: string) => void' is not assignable to parameter of type '(value: string | AxiosResponse<any>) => void | PromiseLike<void>'.
  Types of parameters 'value' and 'value' are incompatible.
    Type 'string | AxiosResponse<any>' is not assignable to type 'string'.
      Type 'AxiosResponse<any>' is not assignable to type 'string'.

341   .then((value: string) => {});
            ~~~~~~~~~~~~~~~~~~~~~

test/typescript/axios.ts:345:9 - error TS2345: Argument of type '(value: string) => void' is not assignable to parameter of type '(value: string | AxiosResponse<any>) => void | PromiseLike<void>'.
  Types of parameters 'value' and 'value' are incompatible.
    Type 'string | AxiosResponse<any>' is not assignable to type 'string'.
      Type 'AxiosResponse<any>' is not assignable to type 'string'.

345   .then((value: string) => {});
            ~~~~~~~~~~~~~~~~~~~~~

TS2345 Error of line 341, 345 is irrelevant to the validateStatus option. So I don't care about this.

[jasonsaayman]

This seems to be a good solution to this issue.