An error cannot be captured within the map function in Javascript

Question

An error cannot be captured within the map function in Javascript

Imagine having a list of users like the following:

var usersList = [
            { firstName: 'Adam', lastName: 'Yousif', age: 23 },
            { firstName: 'Mohamed', lastName: 'Ali' },
            { firstName: 'Mona', lastName: 'Ahmed', age: 19 },
        ];

Now, you want to use the map function on the usersList to create a modified list as shown below :

var returnList = usersList.map((_user) => {
            var _age;
            try {
                _age = _user.age;
            } catch (error) {
                console.log('An error was caught here : ', error); 
                _age = 'FAILSAFE-VALUE'; 
            }
            var obj = {
                firstName: _user.firstName,
                lastName: _user.lastName,
                age: _age
            }
            return obj;
        });

You have implemented a try-catch block within the map function to replace any undefined "age" property of the second user with 'FAILSAFE-VALUE'. However, it is not functioning correctly.

console.log(returnList);
// prints
// [ 
//     { firstName: 'Adam', lastName: 'Yousif', age: 23 },
//     { firstName: 'Mohamed', lastName: 'Ali', age: undefined }, 
//     { firstName: 'Mona', lastName: 'Ahmed', age: 19 } 
// ]

Is there a way to effectively handle errors within the javascript map function?

Your insights are appreciated.

javascript error-handling try-catch undefined

Answer 1

Answer №1

No try catch is necessary in this case:

usersList.map((_user) => {
            return {
                firstName: _user.firstName,
                lastName: _user.lastName,
                age: _user.age || 'BACKUP-VALUE'
            };
        });

Answer 2

No try catch is necessary in this case:

usersList.map((_user) => {
            return {
                firstName: _user.firstName,
                lastName: _user.lastName,
                age: _user.age || 'BACKUP-VALUE'
            };
        });

Answer 3

Answer №2

That's because nothing is thrown (the age is just undefined). To get information about this "error" during the map-operation, you can consider the first part of the code snippet provided. If using try-catch is preferred, the second part throws an error 'manually' when _user.age is undefined. This also demonstrates that try-catch does work within a map-operation.

const usersList = [{
    firstName: 'Adam',
    lastName: 'Yousif',
    age: 23
  },
  {
    firstName: 'Mohamed',
    lastName: 'Ali'
  },
  {
    firstName: 'Mona',
    lastName: 'Ahmed',
    age: 19
  },
];

const getFailSafe = user => {
  if (!user.age) {
    console.log(`Note: user.age not available for ${user.firstName} ${user.lastName}`);
    return `FAILSAFE-VALUE`;
  }
  return user.age;
};

// 1. use a warning if .age not present
const returnList = usersList
  .map((_user) => ({
      firstName: _user.firstName,
      lastName: _user.lastName,
      age: getFailSafe(_user)
    })
);

// 2. throw an Error if .age not present
const returnListWithError = usersList
  .map((_user) => {
    let retVal = {
      firstName: _user.firstName,
      lastName: _user.lastName,
    }
    try {
      retVal.age = _user.age || 
        (() => {
          throw new Error(`ERROR: user.age not available for ${
            _user.firstName} ${_user.lastName} (will continue with 'FAILSAFE-VAULE')`);
          })();
    } catch (err) {
      console.log(`${err.message}`);
      retVal.age = `FAILSAFE-VALUE`;
    }
    
    return retVal;
  });

console.log(returnList.find(v => isNaN(v.age)));
console.log(returnListWithError.find(v => isNaN(v.age)));

.as-console-wrapper { top: 0; max-height: 100% !important; }

Answer 4

That's because nothing is thrown (the age is just undefined). To get information about this "error" during the map-operation, you can consider the first part of the code snippet provided. If using try-catch is preferred, the second part throws an error 'manually' when _user.age is undefined. This also demonstrates that try-catch does work within a map-operation.

const usersList = [{
    firstName: 'Adam',
    lastName: 'Yousif',
    age: 23
  },
  {
    firstName: 'Mohamed',
    lastName: 'Ali'
  },
  {
    firstName: 'Mona',
    lastName: 'Ahmed',
    age: 19
  },
];

const getFailSafe = user => {
  if (!user.age) {
    console.log(`Note: user.age not available for ${user.firstName} ${user.lastName}`);
    return `FAILSAFE-VALUE`;
  }
  return user.age;
};

// 1. use a warning if .age not present
const returnList = usersList
  .map((_user) => ({
      firstName: _user.firstName,
      lastName: _user.lastName,
      age: getFailSafe(_user)
    })
);

// 2. throw an Error if .age not present
const returnListWithError = usersList
  .map((_user) => {
    let retVal = {
      firstName: _user.firstName,
      lastName: _user.lastName,
    }
    try {
      retVal.age = _user.age || 
        (() => {
          throw new Error(`ERROR: user.age not available for ${
            _user.firstName} ${_user.lastName} (will continue with 'FAILSAFE-VAULE')`);
          })();
    } catch (err) {
      console.log(`${err.message}`);
      retVal.age = `FAILSAFE-VALUE`;
    }
    
    return retVal;
  });

console.log(returnList.find(v => isNaN(v.age)));
console.log(returnListWithError.find(v => isNaN(v.age)));

.as-console-wrapper { top: 0; max-height: 100% !important; }

Answer 5

Answer №3

In that scenario, a try...catch block wouldn't be effective because if you try to access a value that doesn't exist, you'll simply get an undefined.

Here's an alternative approach:

var returnList = usersList.map((_user) => {
    return {
        firstName: _user.firstName,
        lastName: _user.lastName,
        age: _user.age ? _user.age  : 'FAILSAFE-VAULE'
    }
});

Answer 6

In that scenario, a try...catch block wouldn't be effective because if you try to access a value that doesn't exist, you'll simply get an undefined.

Here's an alternative approach:

var returnList = usersList.map((_user) => {
    return {
        firstName: _user.firstName,
        lastName: _user.lastName,
        age: _user.age ? _user.age  : 'FAILSAFE-VAULE'
    }
});

An error cannot be captured within the map function in Javascript

Answer №1

Answer №2

Answer №3

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