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‫{\an8}In this video, we will promisify

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‫{\an8}the read file and write file functions.

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‫{\an8}Which means that we will make them so

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‫{\an8}that they return promises

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‫{\an8}instead of us passing a callback functions into them.

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‫So if you want, it's basically a read file function

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‫that returns a promise

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‫and that only receives a file name, no callback.

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‫So let's do exactly that.

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‫So, we're gonna create a read file pro function.

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‫So pro for promise because again

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‫it returns a promise.

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‫And then all we do is to pass in the file name

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‫and as we were saying, return a promise.

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‫So, return new promise.

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‫So we use the promise constructor here

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‫which was introduced into the language with ES6.

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‫So before ES6 like ES5, ES4

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‫and all the previous versions,

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‫there was no support for promises in JavaScript.

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‫But they changed that in ES6

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‫and made the language a lot better in that regard.

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‫So regarding asynchronous JavaScript.

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‫Now this promise constructor here

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‫takes in a so called executor function,

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‫which will get called immediately when promise is created.

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‫And this function will take in two arguments

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‫so resolve and reject.

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‫So resolve and reject,

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‫but for now don't worry for them,

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‫we're gonna be talking about them in a minute.

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‫For now let's just focus on this executor function itself

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‫because this is where we actually do

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‫all this asynchronous work.

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‫In this case the read file work.

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‫So this is where we're gonna call fs.readfile

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‫with the file name that was passed in

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‫and then the usual callback function, okay.

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‫So the error and data,

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‫so just like before.

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‫So keep in mind that we're not actually

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‫changing the read file function

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‫we're simply creating a new function

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‫which behind the scenes of course still runs

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‫the built in read file function

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‫but then returns a promise

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‫so that we can use the promise

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‫instead of the callback function.

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‫Okay.

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‫So we have that read file function here

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‫which will do its work

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‫and when it's ready will come back with the data.

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‫And now this is where the resolve

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‫and the reject functions come into play.

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‫Because both these arguments that are available

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‫in the executor function they are both functions.

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‫Now calling the resolve function

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‫will basically mark the promise as successful

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‫so as fulfilled,

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‫and return the successful value from the promise.

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‫So let's actually do that.

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‫So we call resolve with data.

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‫So whatever variable that we pass into the resolve function

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‫is what will be later available

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‫as the argument in the then method, all right?

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‫So this data here will be the value

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‫that this promise returns to us, okay.

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‫That probably sounds a bit confusing, right?

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‫So again, whatever we pass into this function here,

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‫into the resolve function,

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‫is the result of the promise

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‫that will be available in the then handler.

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‫Now on the other hand,

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‫we can also mark the promise as rejected

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‫in case there was an error, okay.

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‫And we do that by calling,

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‫as you can probably guess,

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‫the reject function.

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‫So, if there was an error

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‫and this is similar to what we did before.

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‫So if there was an error here,

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‫then call the reject function.

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‫And whatever we pass into this one

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‫will be the error that is later available

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‫in the catch method.

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‫So, in this one here.

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‫Okay?

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‫So let's just pass a string in here,

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‫I could not find that file,

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‫some emoji here maybe to make it pop a bit more

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‫in our console.logs, all right?

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‫And, okay.

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‫That is actually already it.

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‫So, let's review this here again.

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‫So we created a new file,

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‫in there we will pass a file name.

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‫So this argument.

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‫And all we do here is to return this promise.

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‫This promise takes in one executor function,

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‫so this one here,

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‫which is where we do our asynchronous work.

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‫In that case here of course that is our

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‫fs.readfile.

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‫Then if we get our data in a successful way

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‫we call the resolve function

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‫with the resolve value.

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‫And if there is an error

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‫we call the reject function, okay?

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‫And so let's now actually use this in practice.

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‫Read file pro,

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‫we pass in our file name.

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‫So that's this one of course

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‫and that's it.

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‫So this will now return a promise.

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‫And so just like before

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‫we can then use our then method on that.

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‫Okay and so this data argument here

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‫will be exactly what we returned from the promise

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‫in case it was successful.

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‫So, with this resolve.

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‫So right now our dog file here

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‫contains labrador

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‫and so this data here will be labrador.

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‫And so that is then later available

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‫in our then handler here.

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‫But of course it doesn't need to have the same name,

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‫I could also call it like result or something

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‫it doesn't matter, all right?

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‫Okay, so let's now take all of this

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‫and actually put it here.

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‫Okay.

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‫Get rid of all of this, save it

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‫and that will run it.

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‫Now we get that error down here

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‫so data's not defined.

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‫That's because I changed it here to a result.

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‫So let's actually go back and change it to data

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‫because here we have data

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‫and here we have data

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‫and so this way I don't have to go ahead

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‫and change all of these variable names.

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‫Give it another save

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‫and here we go.

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‫So it worked, you see?

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‫We actually got the breed,

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‫which is still labrador

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‫and this time from our promise, okay.

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‫So, this really worked.

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‫So our read file pro function

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‫now returns a promise

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‫and that data here is then available on our then handler.

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‫Great, so it's already working

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‫and we can make it a lot better

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‫and we're gonna do that

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‫but for now I also want to promisify

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‫this write file function here.

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‫So basically, do the same as we did

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‫with the read file function.

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‫Now, if you're already know a bit about no js,

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‫maybe you're thinking that there is actually a function node

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‫that will automatically promisify functions for us.

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‫But of course I wanted to show you here

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‫how promises actually work

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‫and how we build them

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‫and so that's why I'm doing all of this here, okay.

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‫So I really want you to learn about promises,

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‫not just how we consume them

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‫but also how to build them.

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‫And so that's why I'm doing it this way.

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‫So, write file and as a promise.

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‫And this one actually doesn't only need the file name

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‫but also the data that should be written to that file.

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‫But then the logic in here is quite similar.

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‫So again, we return a new promise,

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‫in there we have our executor function

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‫which takes resolve and reject as the argument.

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‫And these are kind of standard names

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‫So resolve and reject,

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‫you could call them whatever you really wanted

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‫but this is kind of the standard, okay.

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‫So fs,

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‫write file,

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‫we pass in the file,

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‫the data and then the callback function.

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‫And now something very similar to before,

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‫so if there was an error,

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‫then call the reject function

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‫and pass in some message there.

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‫Could not write the file,

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‫again with this emoji here to make it stand out a bit more

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‫and in case everything worked fine,

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‫well then just resolve.

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‫Now we don't have any data actually to pass in here right?

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‫So we're just gonna pass in some random string

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‫something just like saying success, okay.

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‫So a promise doesn't always

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‫have to return a meaningful value.

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‫In this case all we're really trying to do here

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‫is to write a file

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‫which doesn't really return a meaningful value.

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‫So we're now ready to use this promisified function

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‫but before we do that,

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‫let's actually implement the chaining

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‫that I was talking to you about before.

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‫So right now,

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‫we still in fact have callbacks inside of callbacks

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‫inside of callbacks, right?

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‫So we have this then,

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‫which has this callback.

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‫And then in there there is another then handler

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‫which has yet another callback in it.

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‫So that doesn't really change anything, does it?

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‫Instead we want to chain these then handlers here.

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‫And the secret to doing that

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‫is to make each handler return a new promise.

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‫So how are we gonna do that?

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‫Well, remember that this function here returns a promise.

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‫And so all we have to do is to return

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‫that promise from this first then handler.

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‫Let me show that to you.

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‫So we say return superagent.get

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‫and then the string and that is now a promise, okay.

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‫And then we actually close this callback function here

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‫and chain this then method right onto it.

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‫Give it a save, now we have some error here

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‫and that is on line 40.

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‫Let's get rid of this here

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‫and okay.

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‫It changed the formatting here for us again

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‫that was prettier

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‫and then we actually already have it working down here.

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‫So why does this work again?

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‫Well, of course this function

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‫that we created in the beginning returns a promise.

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‫So on that we can use the then method.

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‫We then make this callback function that's in it

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‫again return a promise.

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‫And like that we can then chain

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‫the next then handler on that.

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‫So this result variable here

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‫will then be the resolved value of this promise

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‫that returned from the previous handler, okay?

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‫Makes sense?

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‫Now to take it one step further

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‫we will go ahead

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‫and use our write file pro function here now as well.

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‫And since we want to keep chaining the then methods

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‫we will again return that actually.

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‫So return, write file pro

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‫pass in the file name, which is this one,

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‫the data...

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‫And that's actually it.

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‫Okay, so we don't need any of this here anymore,

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‫let's just comment it up for now.

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‫Get rid of this space here

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‫and then on there we can chain the next then handler.

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‫So then, and remember that in here

258
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‫we didn't really have any meaningful resolved value.

259
00:11:21,950 --> 00:11:24,240
‫So, we don't want any argument

260
00:11:24,240 --> 00:11:26,580
‫to this callback function here

261
00:11:26,580 --> 00:11:31,433
‫all we want to do is to log this string, right?

262
00:11:32,640 --> 00:11:34,970
‫So now we can actually really get rid of this

263
00:11:36,450 --> 00:11:37,920
‫and okay.

264
00:11:37,920 --> 00:11:39,490
‫So, down here we see now

265
00:11:39,490 --> 00:11:41,090
‫that it's actually already working.

266
00:11:41,090 --> 00:11:43,080
‫And again the trick for being able

267
00:11:43,080 --> 00:11:46,050
‫to chain all of these then methods here

268
00:11:46,050 --> 00:11:49,360
‫is to return a promise before calling each of them.

269
00:11:49,360 --> 00:11:52,360
‫So this one here obviously returns a promise here

270
00:11:52,360 --> 00:11:55,320
‫so then we can chain the then method on it.

271
00:11:55,320 --> 00:11:56,860
‫Then this piece of code here

272
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‫will return a promise

273
00:11:58,110 --> 00:12:00,630
‫and so we can use then on it.

274
00:12:00,630 --> 00:12:02,920
‫Then this piece of code will return a promise

275
00:12:02,920 --> 00:12:07,320
‫and so we can use then on it again, okay?

276
00:12:07,320 --> 00:12:09,010
‫So that is the secret to it.

277
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‫Give it a save again

278
00:12:10,380 --> 00:12:13,500
‫and so down here we see that the breed is still labrador.

279
00:12:13,500 --> 00:12:16,640
‫We see that this here is the link,

280
00:12:16,640 --> 00:12:19,180
‫let's go here and confirm

281
00:12:19,180 --> 00:12:21,300
‫that this is here indeed the same as this.

282
00:12:21,300 --> 00:12:26,050
‫And so it did successfully write the string to the text file

283
00:12:26,050 --> 00:12:30,717
‫using our promisified write file function, okay.

284
00:12:30,717 --> 00:12:34,240
‫And then in the end we have this error handler.

285
00:12:34,240 --> 00:12:38,060
‫And the beauty here is that for all of these chains

286
00:12:38,060 --> 00:12:39,430
‫then handlers in the end,

287
00:12:39,430 --> 00:12:43,020
‫we simply need one single catch handler.

288
00:12:43,020 --> 00:12:44,980
‫So one function handling the errors

289
00:12:44,980 --> 00:12:48,910
‫coming from either one of the three promises, okay.

290
00:12:48,910 --> 00:12:52,090
‫Let's actually only log the error to the console

291
00:12:52,090 --> 00:12:53,900
‫and let's try to, for example,

292
00:12:53,900 --> 00:12:56,160
‫change this file here.

293
00:12:56,160 --> 00:12:57,980
‫Actually not the file,

294
00:12:57,980 --> 00:13:01,940
‫but I'm gonna change the file name here,

295
00:13:01,940 --> 00:13:06,150
‫save it and now we see I could not find that file.

296
00:13:06,150 --> 00:13:07,970
‫So where does that come from?

297
00:13:07,970 --> 00:13:09,720
‫It comes right up here

298
00:13:09,720 --> 00:13:13,410
‫from calling the reject function, okay?

299
00:13:13,410 --> 00:13:16,320
‫So we have an error here because we couldn't find the file

300
00:13:16,320 --> 00:13:18,890
‫and so we called the reject function.

301
00:13:18,890 --> 00:13:22,600
‫And that reject function will then mark this promise here

302
00:13:23,850 --> 00:13:25,720
‫so remember this is a promise.

303
00:13:25,720 --> 00:13:27,780
‫It will mark it as rejected.

304
00:13:27,780 --> 00:13:30,870
‫And therefore, the catch function is called.

305
00:13:30,870 --> 00:13:33,260
‫And so then we log that error to the console.

306
00:13:33,260 --> 00:13:36,313
‫And the result of that is what you can see down here.

307
00:13:37,360 --> 00:13:40,830
‫Okay, let's put it back

308
00:13:40,830 --> 00:13:41,900
‫now it works.

309
00:13:41,900 --> 00:13:43,920
‫Now let's go into the file,

310
00:13:43,920 --> 00:13:45,880
‫create a dog that doesn't exist

311
00:13:50,010 --> 00:13:54,320
‫and now, oh, here we have this big weird error

312
00:13:54,320 --> 00:13:57,583
‫and that's because we're simply logging the entire error

313
00:13:57,583 --> 00:14:00,700
‫and not the error message as we were doing before

314
00:14:00,700 --> 00:14:01,723
‫but doesn't matter.

315
00:14:03,500 --> 00:14:05,203
‫Now give it a save here now,

316
00:14:06,170 --> 00:14:07,220
‫put it back

317
00:14:07,220 --> 00:14:08,500
‫and all right.

318
00:14:08,500 --> 00:14:10,210
‫So now it's all working

319
00:14:10,210 --> 00:14:12,510
‫and we got rid of that triangular shape

320
00:14:12,510 --> 00:14:14,040
‫that we had in our code

321
00:14:14,040 --> 00:14:18,420
‫and now have a so called flat structure of chain promises.

322
00:14:18,420 --> 00:14:20,600
‫So again this is way easier to manage

323
00:14:20,600 --> 00:14:23,150
‫and I hope you can start to appreciate that

324
00:14:23,150 --> 00:14:25,430
‫and it's also more logical to think

325
00:14:25,430 --> 00:14:28,120
‫and to reason about the code like this.

326
00:14:28,120 --> 00:14:29,270
‫Now, in the next lecture

327
00:14:29,270 --> 00:14:31,660
‫we will actually take it one step further even

328
00:14:31,660 --> 00:14:33,570
‫and make this even more readable

329
00:14:33,570 --> 00:14:35,210
‫and easier to use.

330
00:14:35,210 --> 00:14:36,783
‫So stay tuned for that.