Breaking change in validateStatus parameter

Problem:

This is what README.md says about validateStatus parameter:

validateStatus defines whether to resolve or reject the promise for a given HTTP response status code. If validateStatus returns true (or is set to null or undefined), the promise will be resolved; otherwise, the promise will be rejected.

This parameter supports (as specified) undefined in 0.18.x but it does not work properly in 0.19.0 despite the documentation does not warn about this change.

Affects version: 0.19.0 (latest)

Steps to reproduce it: Set validateStatus property to undefined value and request anything that returns 404 Not Found.

Current behavior: It uses the standard validation function (that only accepts 2XX status codes); rejecting any non successful response.

Expected behavior: It should accept any status code received from server as null value does.

Reproducible scenario:

const axios = require('axios'); // v0.19.0

axios
  .get('https://example.com/404', { validateStatus: undefined })
  .catch(console.error);

// output: axios error due to 404 not found

Solution:

Analyze where the bug has been added and fix it.

[sergioregueira]

Indeed, the README.md still says undefined is a valid value for that parameter.

[sergioregueira]

I do not agree #2545 fixes this problem. The behavior for validateStatus was modified from 0.18.x to 0.19.x without any warning (release notes), so this should be the procedure:

1. Restore the previous behavior releasing a patch version (0.19.x) to maintain backward compatibility.
2. Change the behavior to the new one in next minor version (0.20.x) informing the user about this breaking change.