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Okay so we're going to see how
to use functions with type
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script a very important topic.
So it's exactly the same as in
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obviously so we're going to
create a function for example
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sum that's going to receive a
parameter A and parameter B and
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we're just going to return A
plus B so by default if you do
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that let's say you get the
result here of sum and 3
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+ 4.  It's going to work,
but as you can see the result
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is of type any so you don't
know what this is it's not
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typed correctly and this is
because by default the
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parameters that you're sending
here have no types , they have
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the any types so this is
definitely not good because I
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can send something like this
okay and if I do something like
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this I try to console log
results well the behavior is
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not going to be exactly what I
expect as you can see it's 34
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and this is because 3 4 have
been concatinated since A is a
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string that will just be joined
okay instead of being really
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calculated so you really want
to make sure that you're really
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sending a number to have the
expected behaviour so to do
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that it's pretty simple you can
type your parameters doing
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something like this number a
number now it's not possible
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anymore to do something like
this okay it's going to say no
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you're sending a string it's
not assignable to a parameter
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of type number this one So
cool. Now this is protected.
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And now as you can see
automatically the result is now
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of type number absolutely
because here I'm returning a
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number plus a number which
gives a number okay so by
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default if I pass my mouse over
my function as you can see it
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says that this is the function
and at the end you can see it's
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returning something of type
number so it has been correctly
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typed by inference now I'm
going to show you how to send
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parameters that are optionals
and how to send  default values
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to a function. So now I'm going
to remove that. I'm going to
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comment all of this. Now let's
say I create a function that's
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going to be apply discount.
That is going to receive a
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price that is going to be a
number and a percentage of
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discount that should be applied
on this price so it will be
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discount percentage it's also
going to be a number and I'm
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just going to return the price
minus the price multiplied by
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discount percentage divided by
100 okay now I can use it for
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example like this final price
and I'm going to apply a
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discount on a price so for
example the price is going to
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be 300 and I'm going to I don't
know apply 50% discount okay
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and I'm going to log that final
price
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so as you can see yes it's
working I have 50% applied on
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300 cool so if I try to send
something like this it's not
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going to work but now let's say
I work in a company I mean a
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team and someone as to make some
changes in my function, and
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for some reason the developer
here start working on this
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apply function and do some
modification and return
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something else okay for example
this doesn't really make sense
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but you will get the idea. So
what I could have done is
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something like this. I could
have typed my functions
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returned. Okay, I could have
said alright this function is
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always going to return a
number. Now anyone that is
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going to work with this
function will know that oh okay
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it's going to receive a price
that is a number, a discount
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percentage that is a number and
this function is going to
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return a number. So there is no
way I could do something like
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this, okay? So this is going to
help the next developer as well
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and also yourself because if
you do something wrong at some
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point and add a string
somewhere like this where you
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will have an error because this
is going to now be a string and
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it's not going to work okay all
right cool so now let's talk
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about the optional parameters
so in a function you can
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actually send optional
parameters let me give you an
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example let's say you want to
add a parameter that is give to
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charity and it's a boolean if
and if it's true then you're
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going to give one euro of the
price that you have sent here.
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So I'm just going to make a
small change here. It will be a
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new price, I'm going to store
that.
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And if "Give to charity" then I'm just
going to remove one of my
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price and in the end I'm just
going to return your price like
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this now as you can see I'm
missing one parameter okay
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expected argument "3" but got
2 only so you have to add
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give to charity okay so I could
write true and as you can see
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yes it's working but you could
actually make this parameter
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optional you could add here a
question mark and by doing that
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now it's not required anymore
to send this parameter okay
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it's going to work as well and
since I'm controlling here, the
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parameter, everything is going
to work fine. So you can make
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your parameters optionally
here. But be careful, you can
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never have your optional
parameters before required
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parameters, okay? You cannot do
something like this. This is
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not going to work and you will
actually have a message. A
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required parameter cannot
follow an optional parameters.
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All the optionals parameter
should be at the end of your
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parameters. Okay? Now you can
also set a default value for
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your parameters exactly like
you would in JavaScript
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actually so for example I could
do something like this for my
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discount percentage I could say
that by default is going to be
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50 and now it's actually also
going to become an optional
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parameter behind the scene but
it will have a default value if
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not specified so if I do this
you will see that it will still
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be fifty okay as you can see
it's still 50 because by
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default is going to add this
value alright So that is pretty
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much it about the functions,
you just have to remember that
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you have to type all the
parameters that you can make
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then optional or not, you can
send a default value and if you
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send a default value, it's
going to be an optional
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parameter, you can also type
the whole functions, so what is
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going to return, and it's a
more strict approach because it
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will force the next developer to
match the return type, and yes, that
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is pretty much it about the
functions. Okay, so, let's
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continue in the next video and
soon we'll do an exercise.