fetch: Getting cookies from fetch response

From Differences from jQuery section of the Fetch API on Mozilla:

• fetch() won't receive cross-site cookies. You can’t establish a cross site session using fetch(). Set-Cookie headers from other sites are silently ignored.
• fetch() won’t send cookies, unless you set the credentials init option. Since Aug 25, 2017: The spec changed the default credentials policy to same-origin. Firefox changed since 61.0b13.)

The problem turned out to be with the fetch option credentials: same-origin/include not being set. As the fetch documentation mentions this option to be required for sending cookies on the request, it failed to mention this when reading a cookie.

So I just changed my code to be like this:

fetch('/login/local', {
      method: 'POST',
      headers: {
        Accept: 'application/json',
        'Content-Type': 'application/json',
      },
      credentials: 'same-origin',
      body: JSON.stringify({
        username: this.state.username,
        password: this.state.password,
      }),
    }).then(res => {
      return res.json();
    }).then(json => {
      if (json.success) {
        this.setState({ error: '' });
        this.context.router.push(json.redirect);
      }
      else {
        this.setState({ error: json.error });
      }
    });