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So now that we saw how we resolve a promise and handle this value in the then callback, let's reject it.

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So for this, I'll comment out resolve here and instead I'll call reject and reject here, I will also pass

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a value, this time maybe a Javascript object just to mix it up, where I have an error code maybe, let's

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say 500 and a message, an error occurred

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and you're totally free to pass back what you want,

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you could still just pass a string if you want to do that.

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So now we call reject, if we don't change anything else let's save this and let's reload this in the browser

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and let's see what happens.

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We still execute this right after set timeout but then we get an error that we didn't catch this rejected

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value basically. We don't get the console log from before,

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so we don't execute the then blocks here or actually we do but we only handle success cases here in the

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first argument you passed to then. We can also pass a second argument though, you could pass another function

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here which will then handle any rejected promise results, so any errors,

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of course this argument name is also up to you.

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There you could console log err.code and err.messages

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and I know that I can access both properties because I pass them here in this object.

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If we do this and save and we reload our application again, we see this is executed right after set timeout

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again and then after three seconds, we get this log with the error code and our error messages. We

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no longer get this error result from before.

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We also get undefined,

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that's stemming from the second then call where we just don't get the argument because in the error

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case, we don't return a value we could process here.

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Now this is one way of handling promise rejections,

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the more common way is to not use this second argument though. I will quickly comment this out to keep

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it for reference but what I want to show here is that you typically wouldn't use that second argument

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but to make this super easy to read,

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you can simply add catch here,

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so I'll add another method to this chain here, catch, I could add it in a new line but let me stick it right

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to the end of that line

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and here I can now also pass a function which will only be executed in rejected cases,

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so never on resolve and there I can also console log err.code and err.messages.

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The cool thing about catch is it will catch any errors occurring at any step prior to catch,

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so either on the first then block or the second one

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and I could also add catch here right after the first then block and then it would only react to errors

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which were thrown at this step and would ignore errors at the second step,

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so that's possible too. Here I'm now catching errors occurring at any point of time in this

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promise chain

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and if we now save this and reload this application again, we again see executed right after set timeout

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and then we get 500 and an error occured,

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you see that the undefined log is missing because catch caught the error immediately and prevented

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the promise from executing any other lines because we added it at the end here.

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And this is how we can work with promises

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in its very basic feature set, then and catch are the two important methods you have to be aware. Use

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then to use the normal return values in cases where no error occurred and use catch to handle any

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errors which might occur.

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Now with that knowledge, let's go back to the promises we already used in this application and try to

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understand those a bit better.