Props simply are shorthand for properties. Props are how components talk to each other. If you’re at all familiar with React then you should know that props flow downwards from the parent component.
There is also the case that you can have default props so that props are set even if a parent component doesn’t pass props down.
This is why people refer to React as having uni-directional data flow. This takes a bit of getting your head around and I’ll probably blog on this later, but for now just remember: data flows from parent to child. Props are immutable (fancy word for it not changing)
So we’re happy. Components receive data from the parent. All sorted, right?
Well, not quite. What happens when a component receives data from someone other than the parent? What if the user inputs data directly to the component?
Well, this is why we have state.
STATE
Props shouldn’t change, so state steps up. Normally components don’t have state and so are referred to as stateless. A component using state is known as stateful. Feel free to drop that little tidbit at parties and watch people edge away from you.
So state is used so that a component can keep track of information in between any renders that it does. When you setState it updates the state object and then re-renders the component. This is super cool because that means React takes care of the hard work and is blazingly fast.
As a little example of state, here is a snippet from a search bar (worth checking out this course if you want to learn more about React)
Class SearchBar extends Component {
constructor(props) {
super(props);
this.state = { term: '' };
}
render() {
return (
<div className="search-bar">
<input
value={this.state.term}
onChange={event => this.onInputChange(event.target.value)} />
</div>
);
}
onInputChange(term) {
this.setState({term});
this.props.onSearchTermChange(term);
}
}
SUMMARY
Props and State do similar things but are used in different ways. The majority of your components will probably be stateless.
Props are used to pass data from parent to child or by the component itself. They are immutable and thus will not be changed.
State is used for mutable data, or data that will change. This is particularly useful for user input. Think search bars for example. The user will type in data and this will update what they see.
In cross section, a propeller is shaped like a wing to produce higher air pressure on one surface and lower air pressure on the other.
Angle of attack is the angle a wing makes with the oncoming airflow. Pitch angle is the angle a propeller blade makes with its plane of rotation. A wing has nearly the same angle of attack across its entire length. But a propeller blade has a twist, so its pitch angle varies along its length.
On a controllable-pitch propeller, the pitch of the entire blade can be altered during flight to give the best performance at different air speeds. This is similar to changing gears with a car or bicycle.
A ship’s propellers create thrust in water in much the same way an airplane’s propellers create thrust in air.
In order for a propeller blade to spin, it usually needs the help of an engine.