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A Promise represents the eventual completion of an asynchronous operation.
Promises provide a straightforward alternative for composing and managing asynchronous code, without having to write too many callbacks, or spend extra time figuring out what the program should do.
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Promises in JavaScript offer
a more elegant and human readable
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way to manage asynchronous code without
having to write too many callbacks, or
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spend extra time figuring out
what the program should do.
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Promises also give you more
control over your asynch programs.
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What is a promise?
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Well a promise represents the eventual
completion of a asynchronous operation.
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In other words,
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a value that may not be available yet,
like data requested from a server.
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So let's spend a little bit of
time unpacking what all this means
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by first looking at the differences
between callbacks and promises.
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Let's say that,
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instead of making breakfast yourself,
you're in the mood to order breakfast.
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There are two ways you
might order breakfast.
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You can order by phone and
have it delivered, or
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you can order at the restaurant and
pick it up when ready.
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In the first scenario,
you call your favorite breakfast spot and
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place your order, providing them
your phone number and address.
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The order taker says okay, thanks.
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I'll put in your order now and
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call you back to let you know when
the order is ready and on its way.
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And that's it.
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At this point, you can go on and
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do other things while breakfast
gets there, that's convenient.
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But you're also left waiting for
a call back from the restaurant.
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You really don't have control over
when they're going to call you.
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You received nothing in exchange.
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For example,
a ticket number to confirm your order.
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You just wait for them to call and
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deliver your food at around
the time they said they would.
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In this scenario,
you've handed control over whether or
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not you will have breakfast to
the person taking your order.
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You expect it to happen, but maybe they
wrote down the wrong phone number, and
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they can't call back or they call,
but your food never makes it to you.
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Perhaps they even deliver the wrong order.
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Now, most of these problems could
be eventually rectified, but
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it's a messy process.
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A process which hands the control
back to you when, and only when,
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breakfast is delivered successfully.
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This is similar to working
with callback functions.
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You're in some ways handing over
control of your function to a callback
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expecting something else to be called and
executed.
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Like callbacks, the process could
be more susceptible to errors and
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harder to debug if something goes wrong.
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In the second scenario, you go to
the breakfast place, put in your order and
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pay for it.
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The cashier then hands you something back
in return, a receipt with an order number
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and a pager that buzzes to let you
know that your order is ready.
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At that point, you may go off and
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do other things while you wait, like grab
a coffee next door or catch up on email.
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You're still waiting for
the pager to buzz.
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But by just having the pager in hand,
you now have more control over whether or
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not you will have breakfast.
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The pager represents a promise that
something is going to happen as a result
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of ordering breakfast.
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With that being said, the promise is
pending because you're waiting, so
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one of two things could happen.
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The pager buzzes,
your order becomes available to you,
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so you pick it up and breakfast is
fulfilled, or the pager buzzes but
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not because your order is ready.
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Instead, it's to inform you that
your food cannot be made and
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the reason why your order
was in some way rejected.
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This second scenario happens to be more
like how promises in JavaScript work.
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A promise, like a pager,
represents a value that can be
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handled at some point later,
like picking up your order.
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While a callback may leave us with
some uncertainties about that value,
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a promise will promise or guarantee
a future value and nothing can change it,
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regardless of whether or
not it comes back fulfilled or rejected.
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Up next, you will learn how to create
a promise, then we'll continue with
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our people in space project by converting
the callback functions to promises.
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