composition over inheritance inheritance
is when you design your types around
what they are while composition is when
you design your types around what they
do we’re going to learn about
limitations of comparatives and how to
solve them using composition this is a
weekly show when we try to become more
confident and excited about programming
by learning intriguing things that we
didn’t know before a tip is that you
will get more out of this video if you
have watched the previous episode about
factory functions you can do so by
clicking there let’s talk about
inheritance a bit let’s say that we are
designing a game and I need a dope what
after a while we realize that our
software like everything needs cats so
we create a cat class and because nature
calls we add poop to the cat and dog
class that’s duplication we can’t have
that so we lift poop into a shared
animal class and well now we we have a
lot of animals pooping everywhere so we
need a cleaning robot that can drive and
clean we also need a murder robot that
can drive and kill all the cats and dogs
that are pooping all over your nice
Scandinavian white floors
since drive is now duplicated between
cleaning robot and murder robot we
create a robot parent class to put it in
a couple of months of development go buy
your dog your murder robot and friends
have grown into a mature stable system
you’re feeling good about it at this
point in the project without fail the
project manager will say this our
customers demand a murder
robot dog the murder robot dog can kill
drive and bark but it doesn’t have a
digestive system so it cannot whoo and
now we are screwed we simply can’t fit
the murderer bot dog 9
sley into this inheritance hierarchy I
mean I guess that we could create a new
parent object where you put all the
functionality that is share so maybe a
game object with a port method like this
but this means that your objects will
have a ton of functionality that they
don’t use you end up with a sort of
gorilla banana problem where you request
a banana but you get a gorilla holding a
banana and the entire jungle with it or
maybe instead of a parent object we can
just duplicate the functionality like
this we give bark to murder Robo dog in
addition to the dog that is not as bad
but our mothers still wouldn’t approve
thus composition to the rescue again
inheritance is when you design your
types around what they are while
composition is when you design your
types around what they do using
composition we would do this instead a
dog is a poop herb that is also a marker
a cat is a pooper that is also a me our
a cleaning robot is a driver and a
cleaner and a murder robot dog is simply
a driver and a killer and Barker now I
am going to show you how to do this in
practice using javascript we’re going to
do this using normal functions and
normal object literals behold these are
the definitions of Barker and driver
I will let you contemplate these two
functions in solitude for a minute
the astute of you might have noted that
these functions are like the factories
from the previous video but instead of
creating their own state internally they
accept their state as a function
parameter this is so that they can share
the same state have a look at what the
actual murder robot dog Factory looks
like first it creates a state object and
assign some default values to speed and
position it also assigns the name from
the function argument it then uses
something that you might not have seen
before
object assign what object design does is
that it takes an object in this case a
just a new and the object and assigns
the properties from the other objects
into it so in this case it creates a
marker a driver a killer and then merges
them all into the new object and returns
that object we have our murder robot dog
DC a spy object dot assign is a handy
function built into Xmas script six but
all libraries underscore lodash has its
it is also super simple it does almost
nothing so if you’re feeling frisky you
can just write one yourself it’s like
seven lines of code in summary we have
looked at an example of an inheritance
tree the rokda and then we looked at how
to restructure it using composition and
finally we looked at how to actually
compose objects using javascript the
question that is probably on your mind
right now is when do you use inheritance
and when do I use composition well the
vast majority of developers would say
that you should favor composition over
inheritance me personally go one step
further I don’t see the point of using
inheritance at all I don’t think it’s a
good part
and I don’t think it should be used a
lot of people are going to tell you that
if something is an is a relationship
then it should use inheritance for
example me Matias is a map so I can
inherit from the map class if the
relationship is more of a has a nature
such as a car which has an engine then
you should use composition but I think
that reasoning is topsy-turvy while it
is true that I am a man I also have arms
and I also have cooking skills while it
is correct that a car has an engine it
also is a vehicle you can interpret
pretty much any concept both ways when
it comes down to it I think that the
problem with inheritance is that it
encourages you to go predicting the
future inheritance encourages you to
build this taxonomy of objects very
early on project and you are most likely
going to make big design mistakes while
doing that because humans cannot predict
the future even though it feels like we
can and once you build yourself into
that inheritance taxonomy it’s really
hard to get out of it I think it’s just
better to use composition from the start
it’s more flexible it’s more powerful
and it’s really easy to do so why not
and that is it that’s my thoughts on
composition but what are yours comment
down below or maybe just hit like if
you’re not be focus if type in this show
we try to become more confident and
excited about programming by learning
new intriguing things that we didn’t
know before there won’t be an episode
next Monday because I will be at Jay s
Kampf in Berlin if you’re also going to
be there make sure you tweet me at MP
Jamie so that we can say hi in real life
if you’re not going to J’s comm make
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steps until next next Monday stay curious