node js crashing, saying a variable is undefined, when it isnt

Question

Here is my GET request handler, to render a page:

app.get("/:band", async (req, res) => {
   const page = req.params.band;
   try {
   const result = await db.query("SELECT * FROM artists");
   const artistData = result.rows;
   const band = artistData.findIndex((info) => info.onewordid === page);
   const bandInfo = artistData[band];
   console.log(bandInfo);
   const articlesresult = await db.query("SELECT * FROM articles WHERE artist_id = $1", 
    [bandInfo.id]);
   res.render("artistpage.ejs", {
    artistinfo: bandInfo,
    data: artistData,
    articles: articlesresult.rows,
   });
  } catch(err){
    console.log(err)
  }
});

An example band object:

{
  id: 1,
  name: 'The Rolling Stones',
  picadress: 'backgroundpic2.jpeg',
  onewordid: 'stones'
}

The error/output:

undefined
TypeError: Cannot read properties of undefined (reading 'id')
    at file:///Users/admin/Downloads/Web%20development%20Project/Backend/Blog%20Project%20Styling/index.js:83:15
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

Why can I console log this, but I get bandInfo.id as "undefined"?

Answer 1

Try bandInfo[0].id instead of bandInfo.id in the second query.

Answer 2

Consider these four lines of code.

   const artistData = result.rows;
   const band = artistData.findIndex((info) => info.onewordid === page);
   const bandInfo = artistData[band];
   console.log(bandInfo);

I believe the console.log(...) statement is printing undefined into your output, just before the error information is printed into your output.

I also believe the third line assigning to bandInfo is actually incorrectly indexing into artistData, because artistData is a reference to result.rows.

Line 3 which assigns to band is already looking up the target object/row from the results, so you shouldn't need to re-index the rows a second time.

Consider simplifying your code as follows:

   const bandInfo = result.rows.findIndex((row) => row.onewordid === req.params.band);
   console.log(bandInfo);

In this version of the code you do not need page nor artistData, these temporary assignments meant to simplify the code actually increase complexity. That's not a solution to your problem, however. The solution here is to perform the lookup (using findIndex(...)) and assign it directly to bandInfo.

As a debugging aid, before making any other changes, you could change the console.log(...) to give yourself better information:

  console.log({
    page: page,
    band: band,
    bandInfo: bandInfo
  });

Lastly, how to interpret this error:

TypeError: Cannot read properties of undefined (reading 'id')

This means that while trying to read the property id the parent reference was undefined. Given your code, this expression would be the only which could be the problem: bandInfo.id, and again, the sample logging you provided for console.log(bandInfo) is printing undefined before console.log(err) prints the error information.

Good luck!