title: undefined in string form before post data
body: <p>I send data via POST request to NodeJS server.</p>
  
  <pre><code>if(req.method === "POST") {
      let body;
  
      req.on("data", (chunk) =&gt; {
          body += chunk.toString();
      });
  
      req.on("end", () =&gt; {
          console.log(body);
          res.end("OK");
      });
  }
  </code></pre>
  
  <p>The console outputs data, but also <code>undefined</code> in string form.</p>
  
  <pre><code>&lt;input type="text" name="target-url" value="asd"&gt;
  </code></pre>
  
  <p>The output is:</p>
  
  <pre><code>undefinedtarget-url=asd
  </code></pre>
  
  <p>I can write function which removes it, but I think it is way how to fix it.</p>