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- The next two built-in
types I wanna talk about

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are complete opposite,

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and they are the partial
and the required type.

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They just essentially
allow you to create a type

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that either makes everything optional,

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or everything required.

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For example, I have this todo type here

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which has a title and
a completed property.

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And let's say that I have
a form for creating a todo.

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Obviously if I wanted those
form inputs to be in a type,

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they would all be optional,

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because by default the
form is completely blank.

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So to create essentially an
optional version of this todo,

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I can create a brand new type,

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and we'll just call this FormTodo.

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And this is going to be
equal to using our todo,

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but we wanna wrap it in that partial type,

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and we just pass in our todo,

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and now it makes every single key

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inside of this type optional.

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If I hover over this, you
can see title and completed

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are now both optional.

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And this is super simple,
it only takes one generic,

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so it makes the entire object optional.

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Or if we were to use
required here instead,

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and these started out
as optional for example,

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you can now see the FormTodo

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has everything set to required by default,

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and it removes all the optional
things that you can do.

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So it just does it on the entire object,

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every single thing inside of it.

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And if I were to like, do
some nesting, for example,

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I had an address with a street

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which is going to be a string here.

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And let's just make these
different things optional as well.

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When I hover over this,

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you can see that it makes
the address required,

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but it doesn't do deep nesting.

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So this street is still
optional 'cause it's nested.

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It only does the top level keys.

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Now if I wanted to make
only certain keys required

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and not other keys.

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Like I wanna make it so
the title's required,

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but the address can stay as normal,

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well now I need to combine together

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the pick and omit keys with required.

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So for example, here
I want to only make it

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so the address is required or I'm sorry,

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the title is required.

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So from my todo, I can pick specifically

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just the title property.

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Now by doing that, if I close this off,

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I can look here and I
can say I get a form todo

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that just has a required
title, super straightforward.

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But I wanna get all the rest
of the properties here as well.

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So I can use an intersection
to combine this together

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with my original todo.

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And now if I hover over this, you can see

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it's combining these two
different things together.

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But that's going to be
overriding some stuff

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'cause it's gonna be getting
the title from here as well.

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So I want to omit the title from this one.

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And by doing this I'm
essentially saying, okay,

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pick just the title
and make that required.

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Then combine that together
with everything else

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minus the title to give
me what I had before.

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And now if we try to create something

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that has that FormTodo
type, just like that,

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you'll see I can put a
completed variable in here,

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so we'll say that is going to be true,

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and I can pass in a
title which is whatever,

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and you can see our
address is still optional,

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which is great, but our title is required,

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'cause if I remove that, I
get an error because title

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is missing and it is required.

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And that's because of how
I set this up right here.

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So it's very common that
you're going to be seeing

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required and partial together

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with the pick and omit properties to make

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just certain things required

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and just certain things not required.

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It's so common that you may actually

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write your own helper
functions that essentially

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do this line of code for you
inside of like, your own type

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where you can have like,

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PickRequired or RequiredPick
or something like that.

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You can make your own type to
do that if you really want.

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Now the one nice thing about required,

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if we just remove this
back to what we had before

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and we actually get rid of those omit

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and notice it also still works just fine.

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And that's because a required property

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is going to overwrite an optional property

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when you're combining
together two types like this,

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but it will not work as well for partial.

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So for example, by default
our title is required,

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and now we want to make
that title optional.

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You can see here I need to
make sure I omit the title

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from the second one
because required properties

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always override non-required properties.

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Now you can see my title
is optional in this,

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while when I wasn't doing this omit,

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the required title was overriding,

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so now you can see that title is required.

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That's why I'm getting
this error down here.

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So if I wanted to write out a
fancy type to do this for me,

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I'm gonna do it for required, for example.

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What I could do is I could say

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that we wanted to have a required pick.

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And this required pick is
going to be a generic type

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that is going to take
in whatever the thing is

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that I wanna check,
that's just the default,

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T is what I'm going to call this.

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That's the object I'm going to be checking

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or the type I'm gonna be checking.

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And then I want to get
all the keys from that.

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So I can say that I want to get K,

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which is gonna be one of our keys,

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you can call it key if you want.

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And that's going to extend
the key of T just like that.

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So this is very similar to
how pick normally works.

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This is the exact signature pick uses.

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Essentially it's saying we have a type,

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and then we have any of
the keys from that type

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as the required pick section.

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Then I need to set that to an actual type.

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So we can use required here.

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We want to require a specific type,

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but we only wanna pick the
specific keys from here

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that are in the section we called Key.

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So we'll call that Key, just like that.

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There we go.

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And now we have a required pick.

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So if I were to just extract this,

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bring my code back to where
we had our actual todo type.

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And now what I can do down here
is I can say, you know what,

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I wanna make our title required.

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So we'll make it optional up here.

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And then down here, or actually, sorry,

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I'm gonna put this type
in so we can use it.

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Now I'm just gonna call RequiredPick,

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I'm gonna pass in my todo,

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I'm gonna pass in the
thing we want to check,

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which is our title.

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Close that off, and now you can see,

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it's not quite working 'cause
we need to make sure we also

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add in the union or sorry,

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the intersection of our type as well.

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Now if we give that a quick save

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and we hover over our todo,

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you can see property title is missing.

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So it's very common for you
to write a required pick

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that looks like this.

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And if I wanted to do the
same exact thing for partial,

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I could say PartialPick.

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And what that's going to do

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is it's just gonna swap
this out for partial,

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and we need to make
sure on our second one,

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we emit the exact same
properties from our object

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right here, and we called this Key.

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There we go.

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But now if I wanted to
make this a partial pick

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where the title starts out required

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and now becomes optional,

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you can see that that
is working just fine.

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Now to real quickly recap
what these lines of code

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are doing essentially, our required pick,

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we're saying that we have a specific type,

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in our case the todo
type, it doesn't matter,

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just some type, we're going to be

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essentially requiring certain keys from.

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In order to get the keys,

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we're saying that this second property

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can be anything by default.

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If we just said key, it
could literally be anything.

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But by extending the keys
of T, we're saying, okay,

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this must be one of
the keys in our type T.

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In our todo case, that
means it has to be title,

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completed or address.

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You can see here I
actually get autocomplete

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for address, completed and
title, just like I would expect.

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Then what I'm doing is I'm saying, okay,

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we're going to require
picking just from that type

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all of those keys and we're
gonna be intersecting it

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with the initial type just to make sure

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we get all the other
stuff from before as well.

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So it's taking all the
stuff that I'm not picking,

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leaving it the same, and
then the stuff I am picking,

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it's making required.

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It's just like the code we had before

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but this code is a
little bit more condensed

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because we're just using T here

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instead of the full todo name.