Managing JavaScript promise rejections

Question

Managing JavaScript promise rejections

What is the best approach to managing an error, such as the one labeled "new error" in the code snippet below, that occurs outside of a promise?

function testError() {
    throw new Error("new error") // How can this error be properly handled?
    var p123 = new Promise(function(resolve, reject) {
         resolve(123)
    });
    return p123
};

testError().catch(err => {
        return err;  // The execution does not reach here
    })
    .then(ok => {
        console.log(ok)
    });

javascript promise es6-promise request-promise

Answer 1

Answer №1

If you're uncertain about whether a function will throw an error (or return a value) synchronously, you have the option to call it using .then(). This approach will encapsulate the function in a way that ensures consistent handling of the result regardless of how it is generated:

function testError() {
  throw new Error("new error") // how should this be managed?
  var p123 = new Promise(function(resolve, reject) {
    resolve(123)
  });
  return p123
};

Promise.resolve()
  .then(testError)
  .catch(err => {
    console.error(err);
    return err; 
  })
  .then(ok => {
    console.log(ok.message)
  });

Answer 2

If you're uncertain about whether a function will throw an error (or return a value) synchronously, you have the option to call it using .then(). This approach will encapsulate the function in a way that ensures consistent handling of the result regardless of how it is generated:

function testError() {
  throw new Error("new error") // how should this be managed?
  var p123 = new Promise(function(resolve, reject) {
    resolve(123)
  });
  return p123
};

Promise.resolve()
  .then(testError)
  .catch(err => {
    console.error(err);
    return err; 
  })
  .then(ok => {
    console.log(ok.message)
  });

Answer 3

Answer №2

When encountering an error that does not involve async code, it is sufficient to use a traditional try-catch block like the one below:

try {
  testError().catch(err => {
    return err;  // this code won't execute
  })
  .then(ok => {
     console.log(ok)
  });
}
catch(e) {
   // 
}

It's worth noting that as soon as the async-await pattern becomes the primary method for handling promises, the try-catch structure will also become the standard way of managing errors:

try {
    var ok = await testError();
    console.log(ok)
}
catch(e) {
    console.log('e:' + e);
}

This approach effectively deals with both synchronous and asynchronous errors, offering a much cleaner solution compared to using then-catch.

Answer 4

When encountering an error that does not involve async code, it is sufficient to use a traditional try-catch block like the one below:

try {
  testError().catch(err => {
    return err;  // this code won't execute
  })
  .then(ok => {
     console.log(ok)
  });
}
catch(e) {
   // 
}

It's worth noting that as soon as the async-await pattern becomes the primary method for handling promises, the try-catch structure will also become the standard way of managing errors:

try {
    var ok = await testError();
    console.log(ok)
}
catch(e) {
    console.log('e:' + e);
}

This approach effectively deals with both synchronous and asynchronous errors, offering a much cleaner solution compared to using then-catch.

Answer 5

Answer №3

If possible, modify your testError function in this manner

function testError () {
  return new Promise(function (resolve, reject) {
     throw new Error('new error')
     resolve(123)
  })
}

testError().then(ok => console.log(ok),
                 err => console.error(err.message))

Execute it once to observe the error being thrown in console.error
Disable the throw line to witness the promise resolving successfully

Answer 6

If possible, modify your testError function in this manner

function testError () {
  return new Promise(function (resolve, reject) {
     throw new Error('new error')
     resolve(123)
  })
}

testError().then(ok => console.log(ok),
                 err => console.error(err.message))

Execute it once to observe the error being thrown in console.error
Disable the throw line to witness the promise resolving successfully

Answer 7

Answer №4

When errors occur outside of promises, they cannot be caught using promise catch statements.

One alternative is to use a try/catch block to handle the error instead.

function testFunction() {
    throw new Error("A new error has occurred") // How can we deal with this?
    var p123 = new Promise(function(resolve, reject) {
         resolve(123)
    });
    return p123
};

try {
  testFunction().then(result => {
    console.log(result)
  });
} catch (error) {
  console.log(error.message);
}

Answer 8

When errors occur outside of promises, they cannot be caught using promise catch statements.

One alternative is to use a try/catch block to handle the error instead.

function testFunction() {
    throw new Error("A new error has occurred") // How can we deal with this?
    var p123 = new Promise(function(resolve, reject) {
         resolve(123)
    });
    return p123
};

try {
  testFunction().then(result => {
    console.log(result)
  });
} catch (error) {
  console.log(error.message);
}

Answer 9

Answer №5

You should consider rewriting it, as having a caller check for both exceptions and rejections is not a recommended practice:

function handleErrors() {
  return Promise.resolve().then(() => {
    throw new Error("new error"); // this will reject the promise
    return new Promise(function(resolve) {
      resolve(123);
    });
  });
}

handleErrors().catch(err => console.log("Caught " + err));

This behavior is inherent in async functions as they always return a promise:

async function handleErrors() {
  throw new Error("new error"); // this will implicitly reject the promise
  return await new Promise(function(resolve) {
    resolve(123);
  });
}

handleErrors().catch(err => console.log("Caught " + err));

Answer 10

You should consider rewriting it, as having a caller check for both exceptions and rejections is not a recommended practice:

function handleErrors() {
  return Promise.resolve().then(() => {
    throw new Error("new error"); // this will reject the promise
    return new Promise(function(resolve) {
      resolve(123);
    });
  });
}

handleErrors().catch(err => console.log("Caught " + err));

This behavior is inherent in async functions as they always return a promise:

async function handleErrors() {
  throw new Error("new error"); // this will implicitly reject the promise
  return await new Promise(function(resolve) {
    resolve(123);
  });
}

handleErrors().catch(err => console.log("Caught " + err));

Managing JavaScript promise rejections

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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