Handling Promise Rejections

Handling Promise Rejections

A Promise

A brief example of a Javascript promises in action:

const HOW_LONG_TO_MOCK = 500
const thisPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("Success!");
  }, HOW_LONG_TO_MOCK);
});

// USING the promise with the "then" syntax
thisPromise().then(console.log)
// Success!

async function runIt(){
  const promiseResult = await thisPromise()
  console.log(promiseResult)
  // Success!
}

runIt()

This will log "Success!".

A Promise With Rejection

Here is a promise that rejects. the function thisPromise returns a promise.
the runIt() function is async, and awaits the completion of the thisPromise function

const HOW_LONG_TO_MOCK = 500;
function thisPromise() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      reject('My Own Failure Message');
    }, HOW_LONG_TO_MOCK);
  })
};

// USING the promise with the "then" syntax
thisPromise().then(console.log);
// Success!

async function runIt() {
  const promiseResult = await thisPromise();
  console.log('promiseResult')
  console.log(promiseResult)
  
}

runIt()

Running this will return some interesting details in a node terminal session:

node:internal/process/promises:288
            triggerUncaughtException(err, true /* fromPromise */);
            ^

[UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). The promise rejected with the reason "My Own Failure Message".] {
  code: 'ERR_UNHANDLED_REJECTION'
}

Node.js v18.14.2

Node has made their error messages more helful in the recent months/years! Notice that the error includes This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch()..

Handling Promise Rejection With Code

Use The Catch Chained Promise Method

const HOW_LONG_TO_MOCK = 500;
function thisPromise() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      reject(new TypeError('My Own Failure Message'));
      return;
    }, HOW_LONG_TO_MOCK);
  })
};

thisPromise().then(console.log).catch(e => {
  console.log('e?.message')
  console.log(e?.message)
  console.log('do things based on the failure of the promise')
})

Wrap the Calling Of The Promise in a Try/Catch Block

const HOW_LONG_TO_MOCK = 500;
function thisPromise() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      reject(new TypeError('My Own Failure Message'));
      return;
    }, HOW_LONG_TO_MOCK);
  })
};

async function runIt() { 
  try {
    const promiseResult = await thisPromise()
    console.log('promiseResult')
    console.log(promiseResult)
    
  } catch (error) {
    console.log('error?.message');
    console.log(error?.message);
    console.log('do things based on the failure of the promise');
  }
}
runIt()

Running the bove will "hit" the catch block, log the error message, and log the next string.
The above will not log the promiseResult.

Error Properties Are Available

Building an error object that takes advantage of error properties like the error code can be helpful when catching an error thrown by a promise.

Tags:

node

errors