text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
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JORDAN HAYASHI: Yeah. | 5,498.975 | 0.875 |
It's just an array that
has a length attribute, | 5,499.85 | 1.958 |
so we can do this dot state dot
to dos, which grabs that list, | 5,501.808 | 3.572 |
and just do dot length. | 5,505.38 | 2.61 |
And now we have a 0. | 5,507.99 | 1.53 |
And if we add a to do, it
goes ahead and increases. | 5,509.52 | 6.46 |
Cool. | 5,515.98 | 0.5 |
And so how are we going to keep
track of the unchecked to dos? | 5,516.48 | 3.39 |
Well, we know we're tracking every
single to do and whether it's checked | 5,527.55 | 3.78 |
or not. | 5,531.33 | 1.37 |
And so one strategy might
be to get all of the to dos, | 5,532.7 | 3.13 |
remove all of the ones that are checked,
and then count the number remaining. | 5,535.83 | 4.77 |
So what functions might we use
to implement that algorithm? | 5,540.6 | 3.67 |
Yeah, we can use the filter function. | 5,547.889 | 1.541 |
So we can do this dot state
dot to dos dot filter. | 5,549.43 | 4.812 |
And what are we going to filter by? | 5,554.242 | 1.458 |
Well, for each to do,
return if it's not checked. | 5,555.7 | 5.46 |
And then we're left with a new array,
and we can just grab the length. | 5,566.744 | 2.916 |
And so if we do a test,
now we have it unchecked. | 5,572.19 | 2.67 |
What happens when we check it? | 5,574.86 | 1.84 |
Well, that's going to get filtered out
here, and what's left is an empty list. | 5,576.7 | 5.37 |
And you grab the length
of that, and we have 0. | 5,582.07 | 2.54 |
What happens when we delete it? | 5,584.61 | 1.769 |
Well, we don't actually
have to update any logic, | 5,586.379 | 2.041 |
because everything is handled
completely declaratively. | 5,588.42 | 3.627 |
Is there anything here
that says, every time | 5,606.74 | 2.37 |
we change something, make sure to go and
change this count that we're keeping? | 5,609.11 | 4.74 |
Not really, right? | 5,613.85 | 1.02 |
We just assume that we have some to
dos and some data that's coming down, | 5,614.87 | 4.06 |
and we just run calculations
based on that data. | 5,618.93 | 2.51 |
And so we're just declaring
what we want, based | 5,621.44 | 2.28 |
on any data that's passed into us. | 5,623.72 | 2.38 |
And so React, in that
way, is very declarative. | 5,626.1 | 3.515 |
And so this is great. | 5,632.61 | 0.96 |
If we want to add any new features, we
don't really have to worry about, oh, | 5,633.57 | 3.17 |
what is every single
possible method that we | 5,636.74 | 2.56 |
need to update this new feature with. | 5,639.3 | 1.8 |
Whereas if we were doing
this in more Vanilla JS | 5,641.1 | 2.427 |
and more an imperative
nature, we would have | 5,643.527 | 1.833 |
to make sure, oh, if we want to also
keep track of the count of checks | 5,645.36 | 4.21 |
to dos, we're going to have to
check every single method where | 5,649.57 | 3.59 |
that number might be affected
and go ahead and update it there. | 5,653.16 | 2.91 |
And so keeping track of
all that in your mind | 5,656.07 | 3.18 |
might create bugs that
you might not otherwise | 5,659.25 | 3.54 |
if the paradigm were more simple. | 5,662.79 | 1.99 |
And so this is a much more simple way
of saying, hey, given some app state, | 5,664.78 | 4.94 |
go ahead and render an
app based on that state. | 5,669.72 | 3.23 |
Any questions? | 5,675.95 | 2.42 |
Yeah? | 5,678.37 | 0.5 |
AUDIENCE: [INAUDIBLE] | 5,678.87 | 18.795 |
JORDAN HAYASHI: Can you
repeat your question? | 5,697.665 | 1.875 |
AUDIENCE: [INAUDIBLE] | 5,699.54 | 12.51 |
JORDAN HAYASHI: So the
question was if we, | 5,712.05 | 5.1 |
rather than replacing the old
state.todos over here with a new array, | 5,717.15 | 5.88 |
would it still work, if we
were to mutate the array? | 5,723.03 | 3.72 |
And the answer to that is, it depends. | 5,726.75 | 2.144 |
It depends on the React
component that you're using, | 5,728.894 | 2.166 |
and it depends on some
other methods that you | 5,731.06 | 3.07 |
could write that we'll talk
about in future lectures. | 5,734.13 | 4.2 |
But the fact that it
depends is a bad thing, | 5,738.33 | 2.46 |
because then you don't really know-- | 5,740.79 | 1.877 |
it depends on what else you've
written, whether it will work or not. | 5,742.667 | 2.833 |
But if you do it like this, where
it's a completely new array, | 5,745.5 | 2.89 |
it will always work. | 5,748.39 | 2.37 |
And we'll talk about
this in future lectures, | 5,750.76 | 3.14 |
but the React paradigm is
always to do things immutably, | 5,753.9 | 4.53 |
which means if you ever
change something, replace it | 5,758.43 | 2.25 |
with something completely new. | 5,760.68 | 1.41 |
And that-- be a reference to that. | 5,762.09 | 2.22 |
That way, if you're
ever comparing values, | 5,764.31 | 2.82 |
you don't have any bugs that can
appear there if you've mutated a value. | 5,767.13 | 4.23 |
So technically it's changed,
but the reference to that value | 5,771.36 | 3.21 |
has not changed. | 5,774.57 | 3.16 |
Does that make sense? | 5,777.73 | 3.93 |
Cool. | 5,781.66 | 0.5 |
Any other questions about
React, props, state, or anything | 5,782.16 | 3.47 |
that we've seen today? | 5,785.63 | 1.47 |
All right. | 5,792 | 0.78 |
So we talked about to do app.JS
and using React on the web. | 5,792.78 | 8.94 |
But why limit React just to the web? | 5,801.72 | 3 |
So there exists this thing
called React Native, which | 5,804.72 | 2.31 |
is a framework that relies on React
Core and the core algorithms implemented | 5,807.03 | 3.6 |
by the React library. | 5,810.63 | 1.48 |
But it's actually a framework that
allows you to do much, much more. | 5,812.11 | 3.65 |
It allows us to build mobile
apps using only JavaScript. | 5,815.76 | 4.56 |
And so when Facebook released this
framework called React Native, | 5,820.32 | 5.07 |
they released it with the tagline,
"learn once, write anywhere." | 5,825.39 | 2.86 |
So you only have to
learn JavaScript once. | 5,828.25 | 1.924 |
You only have to learn React once. | 5,830.174 | 1.416 |
But you can actually write
it, and it'll run anywhere. | 5,831.59 | 3.53 |
And this supports both iOS and Android. | 5,835.12 | 3.365 |
And so I'll leave you there for
this lecture, and next lecture, | 5,838.485 | 2.625 |
we'll look at this thing
called React Native | 5,841.11 | 1.833 |
and how to run JavaScript
for mobile apps. | 5,842.943 | 4.487 |
[AUDIO LOGO] | 0 | 3.437 |
[? CARTER ZENKE: ?] Well, hello, one
and all, and welcome back to CS50's | 16.72 | 3.36 |
Introduction to Databases with SQL. | 20.08 | 2.4 |
My name is Carter [? Zenke, ?]
and last we left off, | 22.48 | 3.24 |
we learned how to create our
very own database schemas-- | 25.72 | 2.73 |
that is, a way to organize
data in our database. | 28.45 | 3.15 |
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