CODEWITHSUNDEEP
node.jsexpressnode-mysql
D. Winning
D. Winning

Reputation: 312

Node/Express/MySQL - Server crashes on retrieval of rows

I'm pretty new to Express and Node MySQL and I've just been trying to figure a few things out. I have a .get() handler in my app.js file and a button that fetches the handler.

When I log the jsonResponse to the console, it shows me only the first entry - but the app.js seems to be sending all the rows (as logged to the cli console):

// client.js
showUsersButton.addEventListener('click', async () => {
  fetch('/show-users').then(response => {
    return response.json();
  }).then(jsonResponse => {
    console.log(jsonResponse); // first row of table
    contents.innerHTML = jsonResponse; // [object Object]
    contents.innerHTML += jsonResponse.name; // name value for first row in table
  })
})

That is the event handler for clicks to the button. The below is the relevant `.get()' method to deal with this:

// app.js
app.get('/show-users', (req, res) => {
  connection.connect(error => {
    if (error) throw error;

    console.log('Connected to the database.');
  })

  let query = connection.query('SELECT * FROM users');

  query.on('result', row => {
    console.log(row); // logs all entries of the table
    res.send(row);
  })
})

When the button is clicked, two results are displayed to the CLI console - both entries that are in the table (I've set this up just so I can learn). The jsonResponse object is logged to the browser console and it shows as only the first entry in the table - it can display to the page too.

But it crashes the server with:

Server side JS is running.
The server is listening for requests at port 5000.
Connected to the database.
RowDataPacket { id: 1, name: 'Danny', level: 1 }
RowDataPacket { id: 2, name: 'Chloe', level: 1 }
C:\Users\User\desktop\node-site\node_modules\mysql\lib\protocol\Parser.js:80
        throw err; // Rethrow non-MySQL errors
        ^

Error: Can't set headers after they are sent.
    at validateHeader (_http_outgoing.js:494:11)... etc etc

Could anyone let me know what I'm doing wrong?

Upvotes: 0

Views: 267

Answers (1)

sofcal
sofcal

Reputation: 490

You're calling res.send multiple times. The following code is going to run once per result:

query.on('result', row => {
  console.log(row); // logs all entries of the table
  res.send(row);
})

You can only call res.send once per request, because it ends the stream. I don't know the library you're using, but it'll likely emit an end/done and error events

const results = [];
query.on('result', row => {
  console.log(row); // logs all entries of the table
  results.push(result)
})

query.on("end", ()=> res.send(results))

query.on("error", ()=> /* res.send(some error) */)

Upvotes: 1

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