title: fetch() post 404 error
body: <p>I'm writing a simple preact front end for a node.js app. I am trying to read data from a json file in my project directory, take some input from the user, then write it back to the file.</p>
  
  <p>I can load the json file from my project directory using fetch()...</p>
  
  <pre><code>fetch('config.json');
  </code></pre>
  
  <p>... however when I come to write data back into the file, I get a 404...</p>
  
  <pre><code>fetch('config.json', {
      headers: {
          Accept: 'application/json',
          'Content-Type': 'application/json'
      },
      method: 'POST',
      body: JSON.stringify(this.state)
  });
  </code></pre>
  
  <p>Any ideas why this doesn't work?</p>