9/ Errors

Handling Errors

Express has a default error handling
Errors can also be thrown too: throw new Error('Pass required');
Writing error handlers function(error, req, res, next). If an error is thrown anywhere this function will run

app.use((err, req, res, next)=>{
    console.log('Error');
})

define error class in Express

class AppError extends Error{
    constructor (message, status){
        super();
        this.message = message;
        //express default error handling is looking 
        //for a property called status
        this.status = status;
    }
}

module.exports = AppError;

const veryPassaword = (req, res, next)=>{
    const {password} = req.query;
    //http://localhost:3000/?password=test
    if (password === 'test'){
        next();
    }
    throw new AppError('password required', 401);
};

destructuring the error

app.use((err, req, res, next)=>{
    const {status = 500, message = 'Something wrong'} = err;
    res.status(status).send(message)
})

Async errors

Error caused by looking for an id that does not exist in the database
If the function is async, the error handler has to be passed inside the next() parameters

app.get('/products/:id', async (req, res, next) => {
  const {id} = req.params;
  const product = await Product.findById(id);
  if (!product){
    return next(new AppError('Product not found', 404));
  }
  //without return this code will still try run (response is given by the error handler 
  //but Cannot read property 'name' of null will be in the node console log)
  res.render('products/show', {product})
})

Errors from Mongoose or Users

use try-catch (only with async functions)

app.post('/products', async (req, res, next) =>{
  try{
    const newProduct = new Product(req.body)
    await newProduct.save();
    console.log(newProduct);
    res.redirect(`/products/${newProduct._id}`);
    }
    catch(e){
      next(e);
    }
})

use try-catch when throwing an error too:

app.get('/products/:id', async (req, res, next) => {
  try{
    const {id} = req.params;
    const product = await Product.findById(id);
    if (!product){
      throw new AppError('Product not found', 404);
    }
    res.render('products/show', {product})  
  }
  catch(e){
    next(e);
  }
})

Defining an async utility

function wrapAsync(fn) {
  return function(req, res, next){
    fn(req, res, next).catch(e=>next(e));
  }
}

app.get('/products/:id', wrapAsync(async (req, res, next) => {
    const {id} = req.params;
    const product = await Product.findById(id);
    if (!product){
      throw new AppError('Product not found', 404);
    }
    res.render('products/show', {product});
}))

Express 5 is able to handle errors in async functions

Differentiating Mongoose errors

Errors can be of different categories (Validation error, Cast error…)

app.use((err, req, res, next)=>{
  console.log(err.name);
  next(err);
})

we can have some logic for all the errors of a specific category

const handleValidationError = err => {
  //console.log(err);
  return new AppError(`Validation Failed: ${err.message}`,400);
}

app.use((err, req, res, next)=>{
  console.log(err.name);
  if (err.name ==='ValidationError'){
    err = handleValidationError(err); //returns Validation Failed: Validation failed: name: name cannot be blank
  }
  next(err);
})

and in the product schema:

    name: {
        type: String,
        required: [true, 'name cannot be blank']
    },

Written on January 2, 2022