解決了：
@Override

protected void successfulAuthentication(HttpServletRequest req,
                                        HttpServletResponse res,
                                        FilterChain chain,
                                        Authentication auth) throws IOException, ServletException {
    String token = Jwts.builder()
            .setSubject(((org.springframework.security.core.userdetails.User) auth.getPrincipal()).getUsername())
            .setExpiration(new Date(System.currentTimeMillis() + 60 * 60 * 24 * 1000))
            .signWith(SignatureAlgorithm.HS512, SECRET)
            .compact();
    res.addHeader("Authorization", TOKEN_PREFIX + token);
    res.setHeader("Access-Control-Expose-Headers","Authorization");//在這里添加 就OK了
}

參考：
[Axios get access to response header fields Ask Question](https://stackoverflow.com/questions/37897523/axios-get-access-to-response-header-fields)

response.headers.Authorization