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In the last lecture we looked at callbacks as a way to write asynchronous code callback functions are

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really really easy to implement and they're pretty simple callback functions How ever can become quite

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messy and complicated to maintain as we nest functions within functions especially has the level of

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nesting increases.

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So there is another way to write asynchronous code and this is call as the concept of promises callback

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function runs when it is triggered by the calling function.

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So it in effect will wait until it is called by the calling function.

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Similarly a promise is a way to wait for something to happen.

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Promises provide a much cleaner way to write asynchronous code.

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Let's understand this with an example.

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Let's say we write function do stuff

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that takes into arguments data

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and a callback and it simply calls the callback function with a string say the

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we can write this exact same function using promises.

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So let's cut and out this code and see how to rewrite it using promises.

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So we do it like a function do stuff.

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And this time there is a single argument.

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Data will need a callback function when using promises.

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What we are going to do is return a promise a promise is something you can wait on for it to either

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resolve or reject or in other words either succeed or fail.

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So if there is an error during the execution of the function it could reject the promise and if it completes

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successfully you could resolve the wrongs

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first.

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We're going to use the return keyword.

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We have to return the promise.

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So the caller has access to it.

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So we returned a new promise.

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Promise is a predefined construct that takes in our function as an argument

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and this function in turn takes two arguments resolve and reject what what we are doing here is creating

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a new promise and returning it to the caller.

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So the caller can use it as needed.

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So if everything goes well you can call resolve and if there's an error we could call reject.

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So let's say a type of data

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travel equal to encodes Boullion

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and data table equal to

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two

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we resolved that promise.

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Like so otherwise we reject the promise in a similar fashion.

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The triple equal to operator are compared to the left hand side with the right hand side and returns

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true only if the both sides have the same value as well as the same day that I you can't pass any type

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of data as an argument to resolve and reject.

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So you could best ratings numbers booleans or object only thing to remember here is you can't pass only

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one argument.

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So the best way to pass this data is by using an object using an object allows you to pass several items

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or attributes of the data within a single argument.

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So let's pass an object that retains a status and a message string for example.

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So if everything goes fine we resolve the promise using an object that saves status

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it's success and message is

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all is well.

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And if there is an error we reject a promise with an object

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with status

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as editor and message as

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loops.

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There was

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an editor something like that

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and that's come into our the reject far for now and run this function

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we call the function that uses promises just like any other function.

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So our function do stuff takes one argument core data.

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So I'm going to pass in a boolean value.

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That's true so that if block of our function runs so we call the function and then we call then function

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with a dot operator like.

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So dot then which is a function that's just part of the chain and then takes our two callbacks.

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The first one is fine if everything goes well.

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So we call dark then which is a function and pass it our Frist function.

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If everything goes well that simply brings the message in the output.

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Now let's go ahead and run this to see how it goes.

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We should see the Sukses message printed in the output Arsa we can do stuff that doesn't actually do

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anything.

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It simply calls resolve if we pass in a boolean to as input.

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So we see success message on the screen.

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We can do the exact same thing with the errors.

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So let's Ancrum in the reject part of the function the catch this error we must add a second argument

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to our then function and this will receive our error.

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And now we have two functions.

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One when everything goes well and one then something goes wrong.

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So if something breaks said call our function now to simulate a response real fast false or anything

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other than Boolean to to our DO step function and you can see that the error message brings to the screen.

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Another advantage of using promises is there is all and reject functions can only be called at most

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once.

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And contrast read callback functions if you accidentally invoke a callback function twice.

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It will be executed two times.

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However with promises even if you call the resolve or the rejet several times only the first call to

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resolve or reject ever gets executed any subsequent calls to resolve or reject are always ignored in

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the next lecture.

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See how the chain these promises together and call them one after another.

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So let's keep going.