pre-view/CS50-raw · Datasets at Hugging Face

text stringlengths 1 81	start float64 0 10.1k	duration float64 0 24.9
So something special about arrow notation	2,356.37	2.91
is that it has an implicit return, which means we don't actually	2,359.28	3.48
have to write-- so we could have written the same thing as this,	2,362.76	9.142
as a function that takes props.	2,371.902	4.945
And what does it do?	2,376.847	0.833
It returns this.	2,377.68	0.75
So this actually does the exact same thing.	2,391.37	2.529
So we're saying, app two is a function that	2,393.899	1.791
takes a single argument called props.	2,395.69	1.78
And what we do is we return this, and so arrow notation shorthand is just	2,397.47	6.2
you have your arguments in arrow.	2,403.67	2.19
And then if you don't have braces, which referred to it like a code block,	2,405.86	4.56
it just returns whatever's next.	2,410.42	1.65
And so we're saying, return this.	2,412.07	2.22
What is this?	2,414.29	0.93
Well, it's this div and H2.	2,415.22	2.32
We're going to wrap it in a parentheses so we know that it's just one value.	2,417.54	4.91
Does that make sense?	2,422.45	1.47
A great question.	2,423.92	2.56
Do you guys see how these are the same?	2,426.48	2.45
So this is great, but we don't really have all that much power yet, right?	2,433.56	3.71
If we wanted to go ahead and change these props,	2,437.27	2.25
we still have to drop down to this raw JavaScript over here.	2,439.52	4.72
So next we'll see exactly how we create apps that are stateful.	2,444.24	7.597
What does that mean?	2,451.837	0.833
Well, there's this notion of state in React,	2,452.67	2.95
and state is basically an internally managed configuration	2,455.62	3.37
for any component.	2,458.99	3.36
And so now components become classes, and this .state is a property on that	2,462.35	4.365
component's instance.	2,466.715	0.875
So how do we update the state?	2,470.66	1.25
Well, there's a method called this.setState,	2,471.91	5.04
which is implemented in this thing called a React.Component, which	2,476.95	2.82
we have to extend in order to have access to that method.	2,479.77	4.86
And this goes ahead and changes that value.	2,484.63	4.38
So you can pass an object to be merged or a function of the previous state.	2,489.01	3.64
And so if we pass this.setState, an object,	2,492.65	2.53
it will go ahead and merge that in with the existing state.	2,495.18	2.5
So if we pass it in an updater function, it's	2,497.68	2.97
basically a function that gets run when we want to change the state.	2,500.65	3.51
And the set state calls are batched and run asynchronously.	2,504.16	5.65
And of course, any change in state will also cause a re-render,	2,509.81	2.79
because it would be silly if we were to change the state	2,512.6	2.43
but not reflect that in the UI.	2,515.03	3.68
And so how might we go about representing state over here?	2,518.71	3.86
So first, let me copy this so that we can save it.	2,522.57	4
So let's go ahead, and rather than having an app be a function,	2,544.28	3.81
let's actually have it be a class.	2,548.09	2.152
So we can do class app, and we want to extend React.Component.	2,553.61	7.38
And within that, we want to have this method called render, which is	2,567.9	4.68
automatically invoked on a re-render.	2,572.58	4.58
Within render, we want to return this.	2,581.16	3.37
Cool.	2,604.82	0.5
So the way to now write this is rather than having	2,607.84	2.82
app be a function that takes props and returns something,	2,610.66	2.895
we're actually writing a class for app.	2,613.555	2.055
And so as we talked about earlier, classes have instances.	2,615.61	2.79
And React knows that when you want to render something	2,618.4	3.3
like this, if it's a class, go ahead and create a new instance of that	2,621.7	3.6
and pass in these as props.	2,625.3	3.78
And notice how we don't ever take the props anywhere.	2,629.08	2.85
That's because when we extend React Component,	2,631.93	3
React Component, that base class, goes ahead and attaches the props	2,634.93	5.4
to the instance.	2,640.33	0.99
And so in order to get at them, rather than doing props does something,	2,641.32	3.24
we do this dot props dot count.	2,644.56	2.78
So again, the props that come in, in the way that React.Component	2,647.34	3.73
is implemented, it automatically takes the props	2,651.07	2.19
and attaches it to that instance of the class.	2,653.26	3.13
And so in order for us to get them in the render method,	2,656.39	2.72
we do this dot props dot count.	2,659.11	3.93
Does that make sense so far, going from a function to a class?	2,663.04	5.13
We'll talk about this in depth the next lecture.	2,668.17	4.24
And so we talked about this thing called state,	2,672.41	2.63
and how do we actually go ahead and use that?	2,675.04	3.03
Well, when we want to create our state, we actually	2,678.07	2.634
do that in the constructor method.	2,680.704	1.416
And so the first thing that we want to do in our constructor method	2,687.63	3.2
is actually called a super, which means allow	2,690.83	5.61
React.Component to do stuff with the props	2,696.44	2.34
that it would have done otherwise.	2,698.78	2.51
And now go ahead and do what we want to do.	2,701.29	2.244
What do we want to do?	2,703.534	0.916
Well, we want to initialize this thing called state.	2,704.45	2.166
Cool.	2,718.83	0.5
So now we have this thing called state, and how are we	2,719.33	2.85
going to go ahead and update it?	2,722.18	1.98
Well, maybe we should have something called increase count, which	2,724.16	6.9
is a method on this instance.	2,731.06	2.962
And let's go ahead and increase the count here.	2,734.022	1.958
So how might I do that?	2,735.98	1.5
Well, I should call this dot set state and pass in count is this dot state dot	2,737.48	8.25
count plus 1.	2,745.73	3.09
And so now we have a method on this instance called increase count	2,748.82	3.09
that we can call.	2,751.91	0.81
And it should, in theory, increase that count.	2,752.72	3.95
And so rather than referencing the props down here, let's reference state.	2,756.67	3.154
And now it should be 0, and it's just going	2,762.67	2.24
to stay at 0, because we're not doing anything.	2,764.91	3.052
So we can go ahead and get rid of this interval.	2,767.962	2

So something special about arrow notation

2,356.37

2.91

is that it has an implicit return, which means we don't actually

2,359.28

3.48

have to write-- so we could have written the same thing as this,

2,362.76

9.142

as a function that takes props.

2,371.902

4.945

And what does it do?

2,376.847

0.833

It returns this.

2,377.68

0.75

So this actually does the exact same thing.

2,391.37

2.529

So we're saying, app two is a function that

2,393.899

1.791

takes a single argument called props.

2,395.69

1.78

And what we do is we return this, and so arrow notation shorthand is just

2,397.47

6.2

you have your arguments in arrow.

2,403.67

2.19

And then if you don't have braces, which referred to it like a code block,

2,405.86

4.56

it just returns whatever's next.

2,410.42

1.65

And so we're saying, return this.

2,412.07

2.22

What is this?

2,414.29

0.93

Well, it's this div and H2.

2,415.22

2.32

We're going to wrap it in a parentheses so we know that it's just one value.

2,417.54

4.91

Does that make sense?

2,422.45

1.47

A great question.

2,423.92

2.56

Do you guys see how these are the same?

2,426.48

2.45

So this is great, but we don't really have all that much power yet, right?

2,433.56

3.71

If we wanted to go ahead and change these props,

2,437.27

2.25

we still have to drop down to this raw JavaScript over here.

2,439.52

4.72

So next we'll see exactly how we create apps that are stateful.

2,444.24

7.597

What does that mean?

2,451.837

0.833

Well, there's this notion of state in React,

2,452.67

2.95

and state is basically an internally managed configuration

2,455.62

3.37

for any component.

2,458.99

3.36

And so now components become classes, and this .state is a property on that

2,462.35

4.365

component's instance.

2,466.715

0.875

So how do we update the state?

2,470.66

1.25

Well, there's a method called this.setState,

2,471.91

5.04

which is implemented in this thing called a React.Component, which

2,476.95

2.82

we have to extend in order to have access to that method.

2,479.77

4.86

And this goes ahead and changes that value.

2,484.63

4.38

So you can pass an object to be merged or a function of the previous state.

2,489.01

3.64

And so if we pass this.setState, an object,

2,492.65

2.53

it will go ahead and merge that in with the existing state.

2,495.18

2.5

So if we pass it in an updater function, it's

2,497.68

2.97

basically a function that gets run when we want to change the state.

2,500.65

3.51

And the set state calls are batched and run asynchronously.

2,504.16

5.65

And of course, any change in state will also cause a re-render,

2,509.81

2.79

because it would be silly if we were to change the state

2,512.6

2.43

but not reflect that in the UI.

2,515.03

3.68

And so how might we go about representing state over here?

2,518.71

3.86

So first, let me copy this so that we can save it.

2,522.57

4

So let's go ahead, and rather than having an app be a function,

2,544.28

3.81

let's actually have it be a class.

2,548.09

2.152

So we can do class app, and we want to extend React.Component.

2,553.61

7.38

And within that, we want to have this method called render, which is

2,567.9

4.68

automatically invoked on a re-render.

2,572.58

4.58

Within render, we want to return this.

2,581.16

3.37

Cool.

2,604.82

0.5

So the way to now write this is rather than having

2,607.84

2.82

app be a function that takes props and returns something,

2,610.66

2.895

we're actually writing a class for app.

2,613.555

2.055

And so as we talked about earlier, classes have instances.

2,615.61

2.79

And React knows that when you want to render something

2,618.4

3.3

like this, if it's a class, go ahead and create a new instance of that

2,621.7

3.6

and pass in these as props.

2,625.3

3.78

And notice how we don't ever take the props anywhere.

2,629.08

2.85

That's because when we extend React Component,

2,631.93

3

React Component, that base class, goes ahead and attaches the props

2,634.93

5.4

to the instance.

2,640.33

0.99

And so in order to get at them, rather than doing props does something,

2,641.32

3.24

we do this dot props dot count.

2,644.56

2.78

So again, the props that come in, in the way that React.Component

2,647.34

3.73

is implemented, it automatically takes the props

2,651.07

2.19

and attaches it to that instance of the class.

2,653.26

3.13

And so in order for us to get them in the render method,

2,656.39

2.72

we do this dot props dot count.

2,659.11

3.93

Does that make sense so far, going from a function to a class?

2,663.04

5.13

We'll talk about this in depth the next lecture.

2,668.17

4.24

And so we talked about this thing called state,

2,672.41

2.63

and how do we actually go ahead and use that?

2,675.04

3.03

Well, when we want to create our state, we actually

2,678.07

2.634

do that in the constructor method.

2,680.704

1.416

And so the first thing that we want to do in our constructor method

2,687.63

3.2

is actually called a super, which means allow

2,690.83

5.61

React.Component to do stuff with the props

2,696.44

2.34

that it would have done otherwise.

2,698.78

2.51

And now go ahead and do what we want to do.

2,701.29

2.244

What do we want to do?

2,703.534

0.916

Well, we want to initialize this thing called state.

2,704.45

2.166

Cool.

2,718.83

0.5

So now we have this thing called state, and how are we

2,719.33

2.85

going to go ahead and update it?

2,722.18

1.98

Well, maybe we should have something called increase count, which

2,724.16

6.9

is a method on this instance.

2,731.06

2.962

And let's go ahead and increase the count here.

2,734.022

1.958

So how might I do that?

2,735.98

1.5

Well, I should call this dot set state and pass in count is this dot state dot

2,737.48

8.25

count plus 1.

2,745.73

3.09

And so now we have a method on this instance called increase count

2,748.82

3.09

that we can call.

2,751.91

0.81

And it should, in theory, increase that count.

2,752.72

3.95

And so rather than referencing the props down here, let's reference state.

2,756.67

3.154

And now it should be 0, and it's just going

2,762.67

2.24

to stay at 0, because we're not doing anything.

2,764.91

3.052

So we can go ahead and get rid of this interval.

2,767.962

2