Exceptions within promises in JavaScript

Let's now learn how to handle exceptions that happen inside a promise. In the event of such a situation, we must reject the promise using a special reject function, which automatically falls into the second parameter of the promise function:

javascriptlet promise = new Promise(function(resolve, reject) {
  setTimeout(function() {
    ...
  }, 3000);
});

Inside the promise function, we must call resolve if everything went well, or reject if an exception occurred:

javascriptlet promise = new Promise(function(resolve, reject) {
  setTimeout(function() {
    let isError = false;
    
    if (!isError) {
      resolve([1, 2, 3, 4, 5]);   // a promise data
    } else {
      reject('error in promise'); // your error text
    }
  }, 3000);
});

Then, in the then method, you must pass not one, but two functions as parameters: the first will work if the promise fired properly (resolved), and the second - if it fired with an error (rejected):

javascriptpromise.then(
  function(result) {
    console.log(result); // displays a promise result 
  },
  function(error) {
    console.log(error);  // displays an error text
  }
);

As a rule, the above code is written in a more compact way, like this:

javascriptpromise.then(function(result) {
  console.log(result); // displays a promise result 
}, function(error) {
  console.log(error);  // displays an error text
});