title: Why do my React App reload after fetching data?
body: <p>After getting user email and password and getting access to enter, app goes to user profile page and goes after that to sign in form again.
  That happened when I added .then(data => ...). One more issue is that I'm using POST method but after signing in at first time I can see email and password in URL and it stays there after going to sign in form again. And again if I remove .then(data => ...) everything works fine with POST method.</p>
  
  <pre><code>onSubmitSignIn = () =&gt; {
          fetch('http://localhost:3000/signin', {
              method: 'post',
              headers: {'Content-Type': 'application/json'},
              body: JSON.stringify({
              email: this.state.signInEmail,
              password: this.state.signInPassword 
              })
          })
          .then(response =&gt; response.json())
          .then(data =&gt; {
                 if (data === 'success') {
                     this.props.onRouteChange('home');
                 }
          });
      };
  </code></pre>
  
  <p>This function is for Sign In button. .onRouteChange('home') just shows what components should app show.</p>