Is there any problem in handling 'throw error' inside try-catch in async await?

Question

I have a method for posting a product into mongodb using mongoose. I'm using async await instead of the then-catch block. My code:

const Category = require('../models/category');

exports.postProduct = async (req,res,next)=>{
    //const userId = req.user.userId;
    const userId = '5dca886a1ee97b07002de048';
    // const category = req.body.category;
    const category = ['bikes','cars'];
    // const tags = req.body.tags;
    const tags = ['wheels','vehicles','travel','s','s','s'];
    //const imageUrl = req.body.imageUrl;
    const imageUrl = ['https://picsum.photos/200/300','https://picsum.photos/200/300','https://picsum.photos/200/300'];
    try {
    if(tags.length>5){
        const error = new Error('Select only 5 Tags');
        error.statusCode = 406;
        throw error;
    }
    if (!category || category === []){
        const error = new Error('Selected category is empty, select category again');
        error.statusCode = 406;
        throw error;
    }
        const categoryFound = await Category.find({name: {$in:category}}).select('name');
        if (categoryFound) {
            const addProduct = new Product(
                {   name : 'vehicle',
                    description:'Its a good vehicle',
                    categoryId: categoryFound,
                    productImageUrl: imageUrl,
                    creatorId:userId,
                    productRequirement:'buy',
                    tags:tags
                });
          const productSaved = await addProduct.save();
        res.status(200).json({message:productSaved});
        }else{
            const error = new Error('Category not found!');
            error.statusCode = 404;
            throw error;
        }
    } catch (err) {
        if (!err.statusCode) {
            err.statusCode = 500;
        }
        next(err);
    }
};

Errors inside the catch is caught by the express middleware in my app.js.

//Error handling middleware
app.use((error, req, res, next) => {
    const status = error.statusCode || 500;
    const message = error.message;
    res.status(status).json({ message: message, status: status });
});

This works fine. In this specific case, when the array 'tags' are longer than 5, my request from Postman (REST API Dev tool) returns:

{
    "message": "Select only 5 Tags",
    "status": 406
}

When I try to use 'if' checks outside try-catch,I get this error:

 UnhandledPromiseRejectionWarning: Error: Select only 5 Tags
    at exports.postProduct (F:\project\controllers\products.js:17:23)
    at Layer.handle [as handle_request] (F:\project\node_modules\express\lib\router\layer.js:95:5)
    at next (F:\project\node_modules\express\lib\router\route.js:137:13)
    at Route.dispatch (F:\project\node_modules\express\lib\router\route.js:112:3)
    at Layer.handle [as handle_request] (F:\project\node_modules\express\lib\router\layer.js:95:5)
    at F:\project\node_modules\express\lib\router\index.js:281:22
    at Function.process_params (F:\project\node_modules\express\lib\router\index.js:335:12)
    at next (F:\project\node_modules\express\lib\router\index.js:275:10)
    at Function.handle (F:\project\node_modules\express\lib\router\index.js:174:3)
    at router (F:\project\node_modules\express\lib\router\index.js:47:12)
    at Layer.handle [as handle_request] (F:\project\node_modules\express\lib\router\layer.js:95:5)
    at trim_prefix (F:\project\node_modules\express\lib\router\index.js:317:13)
    at F:\project\node_modules\express\lib\router\index.js:284:7
    at Function.process_params (F:\project\node_modules\express\lib\router\index.js:335:12)
    at next (F:\project\node_modules\express\lib\router\index.js:275:10)
    at F:\project\app.js:54:5

Is this way of throwing error inside try-catch for checking length, checking empty array using if statement efficient? Is there any better way to do it?

Answer 1

You are throwing an error but there is no one to catch it. When you create an async function, it returns a promise. So, when you are throwing an error but not catching it, the function is returning a rejected promise. That is why you are getting UnhandledPromiseRejectionWarning . When you are using async/await you must catch the promise rejection using try-catch.