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In this video, we'll create a stateful component by first defining an initial state inside the Counter class.
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In this video, we're going to create
a stateful component by first defining
0:00
an initial state inside the counter class.
0:04
You'll learn that in React
state changes over time,
0:07
usually in response to users action.
0:11
So what part of the counter will
change when users interact with it?
0:13
The score will increase or decrease
depending on which button they click.
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Score is our state.
0:22
So now let's go into our Counter
component and start creating state.
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Since state is an object, you create and
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initialize state within a class
inside the constructor method.
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So first, I'll set up a constructor.
0:35
Again, if you're not familiar with classes
and constructor methods, be sure to review
0:38
the Tree House videos posted in
the teacher's notes with this video.
0:42
Inside the constructor, I'll call super in
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order to call the constructor of
the component class that we're extending.
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And this needs to be done before
we can use the this keyword
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within the constructor.
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Then to initialize our component state,
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write this.state and
set it equal to an object.
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You must name this object state
otherwise this will not work.
1:09
Since state is data that changes over
time, we first need to set an initial
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state or the state of
the component when it first bounce.
1:19
The state in our counter is going to
be the score we want to display for
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each player.
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I'll call the state score and
set it to 0 by default.
1:29
Now that we've initialized state,
over end React dev tools,
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select a counter component
inside a player component.
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And you should see a new property
called State up here in the right pane.
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We see the course state and
it's value is 0.
1:45
Next, let's remove the score
prop being passed to
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the counter and player components.
1:54
Because Counter is now
maintaining its own score state,
1:58
it doesn't need the score information
from its parent player component.
2:01
You access state in a similar
way to how you access props.
2:06
In the Counter components render method,
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replace this.props.score
with this.state.score.
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You can think of the render
method in a class component
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as being a function of not just props,
but props and state.
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In other words,
if either props or state changes,
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React executes the render method to
update what gets displayed to the user.
2:30
Now once we refresh our app,
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we see that all the players scores
are set to the initial state of 0.
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We can even change the score
state of a counter component
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instance in React Dev Tools and
see the score change on the page.
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Great, so you've just built your
first stateful react component.
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Before we move on,
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I want to teach you a second way you can
initialize state inside a React component.
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A shortcut,
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you'll often see state initialized
inside a class constructor like this.
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But you can also initialize state
directly inside the class definition,
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using a class property.
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You omit the constructor method and
super all together, and
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reference the state property directly.
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You don't need to write it as this.state,
just state, and
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set it equal to the object.
3:30
This class properties syntax is a feature
of JavaScript that's currently not
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supported in all browsers.
3:39
Since we're using the Babel
transpiler in our app,
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we don't have to worry
about browser's support.
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Babel will transpile our code and add
a constructor for us behind the scenes.
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The way you initialize state is up to you.
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I prefer the class property syntax because
it allows for a cleaner class component
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and I don't have to remember to
setup a constructor and call super.
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So that's what I'm gonna
be using moving forward.
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All right,
we've moved our score into a state object
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instead of getting the score from props.
4:10
But so far,
it doesn't seem to have a benefit.
4:12
Up next, you'll learn how to modify
state from inside a component,
4:15
something you're not
able to do with props.
4:18
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