- What is a Promise? 3:36
- Create a Promise 4:43
- Reject a Promise and Handle Errors 3:25
- Promises Review
- Create a Promise Review 6 questions
- From Callbacks to Promises 5:59
- Handle Multiple Promises with Promise.all 3:35
- Before You Continue
- Perform Cleanup With finally() 2:47
- Using Fetch 6:21
- JavaScript Promises Review 6 questions
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Reject a Promise and Handle Errors
3:25If 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().
Now I'll teach you how to
handle a rejected Promise.
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If an error occurs, the Promise status
changes from pending to a rejected, or
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failed state.
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The good thing is that promises give you
the opportunity to handle the error with
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a method called catch.
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I'll start by changing resolve to reject.
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And pass it the string, No!
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There was a problem with your order.
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Now I can call the catch
method on breakfastPromise,
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which deals with rejected cases only.
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I can do that by chaining it
to the then method, like so.
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The function passed to catch takes one
argument, which is the rejection reason,
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or the error message to display.
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I'll pass catch an arrow function
that takes the parameter, err, and
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logs it to the console.
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I'll Save my file and
run it in the terminal.
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The reject method changes the status of
the Promise from pending to rejected.
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So this time, instead of then,
catch gets called,
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invoking the function passed to it and
in the Console we see the message, No!
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There was a problem with your order.
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Keep in mind that if you
don't specify reject and
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a catch method to handle a rejected
Promise, the Promise will fail silently.
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Errors will be swallowed up with no
side effects to the rest of your app.
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In other words, the program will continue
to execute as if everything was fine.
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Now that you know how resolve and reject
work, I'm going to write a conditional
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statement inside setTimeout that checks
the true and false value of an order.
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Let's have it say if order is true resolve
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this promise to the value
Your order is ready.
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Come and get it!
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Else, if it's false,
reject the Promise and
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set the rejection reason to
Your order cannot be made.
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This time I'm passing the Error object
to reject versus only a string.
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While it's not required, it does help
with debugging because it will display
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the stack trace alongside
the rejection reason.
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All right, so now to test this,
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I'll initialize a variable
named order to true.
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I'll run the file in the Console,
since order is true,
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the Promise resolves to
Your order is ready.
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Come and get it.
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Now, I'll change order to false and
run the file again.
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This time the Promise is rejected,
so the rejection reason,
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Your order cannot be made, displays in
the Console, followed by the stack trace.
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Good, finally to make your
code more readable and
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the Promise sequence easier to follow,
you can place each then and
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catch call on a separate line like this.
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This approach will come in handy
starting in the next step,
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where you'll chain together
a sequence of Promises.
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