Promise not triggering .then() after fetch Express API/Mongoose call

Question

I'm hoping someone can tell me why I'm failing to return a promise and call .then() on it.

I'm making an inventory system for my Pokemon cards with a react front-end and an express backend. When I hit the "increase inventory" button on the FE, here's the function that calls the API:

incInventory(card, cond) {
  // this.setState({currentValue: this.state.currentValue + 1});
  const result = fetch('http://mySecretAPIServer/items/' + card._id, {
    method: 'PUT',
    headers: {
    'Content-Type': 'application/json',
    'Accept': 'application/json',
    },
    // We convert the React state to JSON and send it as the POST body
    body: JSON.stringify({_id: card._id, cond: cond, op: "incQuantity", otherId: card.id})
  });
  console.log(result);
  // ***WHY ISN'T THIS RUNNING?!?!?!?***
  result.then((res) => {
    console.log("this isn't printing...");
  });
}

Here is the Express endpoint it's hitting:

// UPDATE (Modify)
router.put('/:id', (req, res) => {
    const conditionToUpdate = "q" + req.body.cond;
    const amtToUpdate = req.body.op === 'incQuantity' ? 1 : -1;

    return new Promise((resolve, reject) => 
        Pokemon.findByIdAndUpdate(req.params.id, {$inc: {[conditionToUpdate]: amtToUpdate}}, (err, result) => {
            resolve(result);
        }));
});

As you can see, I'm trying to return a promise from the endpoint. After the fetch on the FE, here is what the promise looks like before trying to call .then() on it:

But .then() never runs. I don't even need the data from the fetch, really---i just need to trigger some code after the fetch is complete.

Hope someone can help me understand what I'm missing! Thanks!

Answer 1

You're returning the promise and resolving it, but you aren't sending any response back to the client with the res.

In the express code, you don't need to have a promise, you can do this:

router.put('/:id', (req, res) => {
    const conditionToUpdate = "q" + req.body.cond;
    const amtToUpdate = req.body.op === 'incQuantity' ? 1 : -1;

    Pokemon.findByIdAndUpdate(req.params.id, {$inc: {[conditionToUpdate]: amtToUpdate}}, (err, result) => {
        res.json(result);
    }));
});