function appropriate JavaScript if you
have not watched the two previous
episodes you should do so first before
watching this one can do so by clicking
there ish in this episode we will learn
about yet another higher-order function
read dupes so why should you learn
reduce for me learning about reduce was
one of those no holy moments in
programming I had heard about reduce I
heard the word I kinda sorta thought it
was something that you could doing
databases no sequel databases and when I
finally learned it oh my god I can use
this everywhere then you do your
interview should so what is reduce
before we talk about that let’s recap
what we learned in previous videos we’ve
learned about a couple of higher-order
functions map filter and reject what all
these have in common is that they
transform a list into something else
map will take an array and transform
that into an array of the same length
but with each individual item transform
filter transforms an array into a
smaller array we also briefly learned
about reject which does the same thing
as filter but inverted another example
that I haven’t mentioned is find which
does the same thing as filter but only
returns the first item so that
transforms an array into a single item
there are lots of array transformations
like this on the array object but what
to do if you can find one that fits that
is where reduce comes in
map filter reject and find are all list
transformations they turn your list into
something else but they are all pretty
specific reduce is not reduce is the
multi tool on list transformations it
can be used to express any list
transformation in fact you can use
reject to implement functions like math
further reject or find or any other list
transformation reduce is the super list
transformation that you can fall back on
if you can’t find a prebuilt
list transformation that fits your
problem and now we have done three
minutes without code which means that
you guys have died of boredom and I’m
talking oh oh okay
pause the video do some stretching oh my
god it’s cool have a look at this ring
our mission is to summarize or the
amounts I’m going to show you how to do
this use reduce but first let me make a
for loop let’s have a look at this we
create a variable called total amount
and we loop through the orders and for
every order here we take the amount and
add it to total mom says console.log
run it and it gives us this up its
reimplemented s using reduce I will
comment out the for loop so that it can
compare them here we go so just like map
or filter reduce is a function on the
array object and just like math and
filter it takes a callback function
but unlike map and filter it wants an
object you can think of this object as a
starting point for our sum it’s going to
be zero and this sum is going to be
passed as the first argument to the
reduce code just like map and filter
reduce will also receive the iterated
item but it’s going to be the second
argument now what we going to do is that
we’re going to take the sum that we
received the initial value and we’re
gonna add the order amount to it so this
is going to be take the first order and
add zero point and return and this
return value will in turn be passed as
the sum in the next situation which in
turn will add its amount to it and
return that and so on until we are
finished so if we save and run it will
give us the same amount as the for loop
let me place a console dot log inside
hello where we echo out the Sun and the
order that we’re getting in and we run
it the first situation we are receiving
the first object in the array and we’re
also getting the starting value that we
are passing in as a second argument to
reduce we return a value here which will
be zero plus 250 which will be passed on
to the second iteration of reduce which
we see here so the return value is
passed in as a sum along with the second
item in the array and we’re adding on
400 to 250 and we’re returning that to
the next iteration of the loop which be
what will be 650 and it will get the
third item and so on until we have
iterated all the items in the array and
then the final return value will be
returned and that will be our self let’s
compare this to the board will remove
the consult long non-supportive doesn’t
happen also look long if we look at the
for loop it is 97 characters if you look
here down in the lower left corner and
when I select the reduce function you’ll
see that it’s 88 characters it is
slightly shortened the difference is not
quite as dramatic as between the for
loop and map and filter when you can use
those because they are very specific and
very tailored to their job so you have
to give reduce some slack because it’s a
lot more generic and to do a lot more
work in I could talk about it can do any
array transformation which we will look
at later but reduce is still
shorter than the the for-loop and
especially remember that we talked about
arrow functions new format for functions
you’ll see that it is a lot so in this
video I talked about how reduce fits
into your toolbox how math and filter
and reject and find are all list
transformations and how reject is the
multi tool of list transformations which
you can use if you can find a pre-built
transformation that fits your purposes
I’ve also showed you a super basic
example of what you can use reduce for
however the example that I showed you is
extremely basic and really doesn’t
showcase the extreme power of this
function so I want to spend the next
episode on reduce as well talk about how
reduce combines with other list
transformations I want to talk about how
reduce can create much more complex and
interesting objects than just the sum
that I showed you in the example I am
going to sell you on how powerful this
function is do not mess it follow me on
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