React State

React components (functions) has a built-in state object. The state object is where you store property values that belong to the component. When the state object changes, the component re-renders.

Creating the state Object
The state object is initialized in the constructor in the following script. In JavaScript, super refers to the parent class constructor. (In this script, it points to the React.Component implementation.) Importantly, you can’t use this in a constructor until after you’ve called the parent constructor. 

constructor( props ) {
  super( props );
  this.state = { brand: "Ford" };
}

The state object can contain as many properties as you like: 

constructor( props ) {
  super( props );
  this.state = {
    brand: "Ford",
    model: "Mustang",
    color: "red",
    year: 1964
  };
}

Using the state Object
Refer to the state object in the render() method anywhere in the component by using the this.state.propertyname syntax. 

render( ) {
  return (
    

      
My { this.state.brand }

      

        It is a { this.state.color } { }
                { this.state.model } { }
           from { this.state.year }.
      

    

  );
}

Changing the state Object
To change a value in the state object, use the this.setState() method. When a value in the state object changes, the component will re-render, meaning that the output will change according to the new value(s). JavaScript arrow functions make use of the fat arrow (=>) syntax to define functions. From the traditional way of declaring functions in JavaScript: 
   function( ) { ... }
you can move on and use the syntax you can see in the following: 
   const ( ) => { ... }
The following example adds a button with an onClick event that will change the color property: 

changeColor = ( ) => {
  this.setState( { color: "blue" } );
}
render( ) {
  return (
    

      
My { this.state.brand }

      

       It is a { this.state.color } { }
               { this.state.model } { }
          from { this.state.year }.
      

      <button
          type    = "button"
          onClick = { this.changeColor }
        >  Change color
      </button>
    

  );
}