Constructor

The constructor in React JS is a special method used for creating and initializing an object. This method is called automatically when a class is instantiated. It's a great place to set the initial state and other initial values because it's the first method to run when a component is being created.

In React JS, a constructor is primarily used to bind event handlers to the component class and/or to initialize the local state of the component.

Here's a simple code snippet that shows how to use a constructor in React JS:

```jsx
class MyComponent extends React.Component {
constructor(props) {
super(props); // Calls the constructor of the parent class (React.Component)
this.state = { name: '' }; // Initializing the state of the component

this.handleNameChange = this.handleNameChange.bind(this); // Binding the event handler to the component
}

handleNameChange(event) {
this.setState({name: event.target.value}); // Updating the state
}

render() {
return (
<input type='text' onChange={this.handleNameChange} /> // Using the event handler
);
}
}
```
In the code above, the constructor is defined with the parameter “props“ to pass any props down to the parent constructor (React Component). The 'super(props)' statement calls the parent class's constructor so that it can initialize itself. The constructor also initializes the state of the component, as well as binds any event handler to the component.

The 'state' holds values local to the component. 'handleNameChange' is an event handler used to update the state of 'name' when the input is changed. It's worth noting that event handlers must be bound to their component instances using the .bind() method in the constructor. This is to ensure 'this' is defined whenever the methods are invoked.

This is a basic example, actual usage can be more complex depending on the needs of your specific component or application.