To keep the current behavior it will be necessary to use the paramsSerializer: 'simple' option. (depth arrays of 1, object json.stringified, skip null values)

If you just want to use brackets you can use the paramsSerializer: 'brackets' option.

The default formatting is now more complete and corresponds to qs.

The ideal would be to be able to remove the compatibility options and return to the documentation of a library as qs to use paramsSerializer as soon as the configuration is no longer the default one.

When an object is passed as paramsSerializer, it must be passed as options to encodeParams.

And we should add the option arrayFormat: 'repeat' (to fix #1443 like qs)

axios.get(
  "/url",
  { params: { a: ["b", "c"] } },
  { paramsSerializer: { arrayFormat: "indices" } }
);
// '/url?a[0]=b&a[1]=c'

axios.get(
  "/url",
  { params: { a: ["b", "c"] } },
  { paramsSerializer: { arrayFormat: "brackets" } }
);
// '/url?a[]=b&a[]=c'

axios.get(
  "/url",
  { params: { a: ["b", "c"] } },
  { paramsSerializer: { arrayFormat: "repeat" } }
);
// '/url?a=b&a=c'

On this I don't think this should be implement as mentioned previously if you'd like to use another format, you can set a custom paramsSerializer as follows:

axios.defaults.paramsSerializer: function(params) {
  // return a query string
}

Using the qs library:

axios.defaults.paramsSerializer: function(params) {
  return qs.stringify(params, { indices: false }); // param=value1&param=value2
}