How to get res json value in Express.js

Question

I am newbie in node.js. I am trying to do something like this -

I have created one Add To Cart API where i am first checking whether session cart exist or not. If not exist, i am creating new cart via getCartCode(res) method.

app.post('/addToCart', function(req, res) {
console.log('[POST] /addToCart');

var cart = req.body.conversation.memory['cart'];

if(!cart) {
   const response = getCartCode(res);
   console.log("******* Result ************ : " + response.conversation.memory['cart']); // Giving undefined exception here
}
}

getCartCode - This method create cart and return the code which i am returning via res.json

function getCartCode(res)  {

return createCart()
  .then(function(result) {
    res.json({
      conversation: {
        memory: {
          'cart': result,
        }
      }
    });

    return result;
  })
  .catch(function(err) {
    console.error('productApi::createCart error: ', err);
  });
 }

Now what i want is, i want to get cart code in addToCart API in response. I am trying to print cart code in console.log, but it's not printing anything and throwing exception.

Answer 1

First, you're trying to call a function that returns a Promise, which is asynchronous, and expect it to behave as if it was synchronous, which is not going to work:

// this won't work
const response = getCartCode(res)

function getCartCode(res) {
  return createCart().then(function(result) {
    return {...}
  });
}

You'd have to use something like async/await if you want to be able to use getCartCode similarly to what you're doing right now, like so:

app.post('/addToCart', function(req, res) {
  async function handleAddToCart() {
    // i suggest you use something like the `lodash.get`
    // function to safely access `conversation.memory.cart`
    // if one of these attributes is `undefined`, your `/addToCart`
    // controller will throw an error
    const cart = req.body.conversation.memory.cart

    // or, using `lodash.get`
    // const cart = _.get(req, 'body.conversation.memory.cart', null)

    if (!cart) {
      const response = await getCartCode().catch(err => err)
      // do whatever you need to do, or just end the response
      // and also make sure you check if `response` is an error
      res.status(200).json(response)
      return
    }
    // don't forget to handle the `else` case so that your
    // request is not hanging
    res.status(...).json(...)
  }
  handleAddToCart()
})

function getCartCode() {
  return createCart()
    .then(function(result) {
      return { conversation: { memory: { cart: result } } }
    })
    .catch(function(err) {
      console.error('productApi::createCart error:', err);
      throw err
    })
}

Second, don't pass res to the createCart function. Instead, get the data you need from the createCart function and call res.json inside the /addToCart controller.

Here's how you have to handle this:

app.post('/addToCart', function(req, res) {
  const cart = req.body.conversation.memory.cart

  if (!cart) {
    getCartCode()
      .then(function (result) {
        res.json(result)
      })    
      .catch(/* handle errors appropriately */)
     return;
  }
  // return whatever is appropriate in the `else` case
  res.status(...).json(...);
})

function getCartCode() {
  return createCart()
    .then(function(result) {
      return { conversation: { memory: { cart: result } } }
    })
    .catch(function(err) {
      console.error('productApi::createCart error:', err);
      throw err
    })
}