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Asynchronous Programming with JavaScript

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Reject a Promise and Handle Errors

3:25 with Guil Hernandez

If an error occurs, the promise status changes from "pending" to a "rejected" (or failed) state. Promises give you the opportunity to handle the error with a method called catch().

Running a JavaScript file with the node command

If you are not able to run the file promises-breakfast.js in your Terminal or console using the command node promises-breakfast.js, you can link the file to index.html and then test the output in your browser's console:

<body>
  ...

  <script src="js/promises-breakfast.js"></script>
  <script src="js/callbacks.js"></script>
</body>

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    Now I'll teach you how to handle a rejected Promise. 0:00
    If an error occurs, the Promise status changes from pending to a rejected, or 0:03
    failed state. 0:08
    The good thing is that promises give you the opportunity to handle the error with 0:09
    a method called catch. 0:13
    I'll start by changing resolve to reject. 0:15
    And pass it the string, No! 0:20
    There was a problem with your order. 0:23
    Now I can call the catch method on breakfastPromise, 0:30
    which deals with rejected cases only. 0:33
    I can do that by chaining it to the then method, like so. 0:36
    The function passed to catch takes one argument, which is the rejection reason, 0:43
    or the error message to display. 0:48
    I'll pass catch an arrow function that takes the parameter, err, and 0:50
    logs it to the console. 0:54
    I'll Save my file and run it in the terminal. 0:59
    The reject method changes the status of the Promise from pending to rejected. 1:03
    So this time, instead of then, catch gets called, 1:08
    invoking the function passed to it and in the Console we see the message, No! 1:11
    There was a problem with your order. 1:16
    Keep in mind that if you don't specify reject and 1:18
    a catch method to handle a rejected Promise, the Promise will fail silently. 1:22
    Errors will be swallowed up with no side effects to the rest of your app. 1:27
    In other words, the program will continue to execute as if everything was fine. 1:32
    Now that you know how resolve and reject work, I'm going to write a conditional 1:40
    statement inside setTimeout that checks the true and false value of an order. 1:45
    Let's have it say if order is true resolve 1:51
    this promise to the value Your order is ready. 1:56
    Come and get it! 2:03
    Else, if it's false, reject the Promise and 2:06
    set the rejection reason to Your order cannot be made. 2:11
    This time I'm passing the Error object to reject versus only a string. 2:20
    While it's not required, it does help with debugging because it will display 2:26
    the stack trace alongside the rejection reason. 2:30
    All right, so now to test this, 2:32
    I'll initialize a variable named order to true. 2:36
    I'll run the file in the Console, since order is true, 2:42
    the Promise resolves to Your order is ready. 2:45
    Come and get it. 2:48
    Now, I'll change order to false and run the file again. 2:49
    This time the Promise is rejected, so the rejection reason, 2:56
    Your order cannot be made, displays in the Console, followed by the stack trace. 3:00
    Good, finally to make your code more readable and 3:05
    the Promise sequence easier to follow, you can place each then and 3:09
    catch call on a separate line like this. 3:14
    This approach will come in handy starting in the next step, 3:19
    where you'll chain together a sequence of Promises. 3:22

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