State is the fundamental way to store and retrieve your React application’s data. It’s nothing but a JavaScript object, which React allows you to update via the setState() method.

Using setState() is pretty straightforward, simply call the method and pass in the state variables that need to be updated, which in turn, re-renders the component tree.

setState({ loading: true });

Since setState() updates the state of a particular React component, it needs to be called within that component’s context.

class InputBox extends React.Component {

... submitHandler() {

this.setState({ loading: true });

} ...

}

In the example above, this provides the context to the setState() call, which refers to the InputBox component.

Note : setState() is actually asynchronous. More info here.

As mentioned earlier, this represents the component itself. If you log it out, you’ll see that it returns the state and props tied to the component.

class InputBox extends React.Component {

... componentDidMount() {

console.log(this);

}

}

Logging out this returns the whole React component

Now this means that updating the state of a React component can be as easy as this -

InputBox.setState({ loading: true });

A suitable method to update the state of a Parentless component would be to pass a callback as a prop, to the Child component, which once fired, updates the state of the Parent.

Pretty handy! But this obviously makes sense when you have a relatively smaller component tree, and you would want to avoid returning callbacks from a deeply nested component, all the way up to the topmost parent.

But what if we want to update the state of the topmost parent, from a deeply nested component ?

Well, we can keep a reference of the topmost parent’s component instance, and call setState() when we want to trigger the state change.

Let’s see how -

Note : ReactDOM.render() returns a reference to the component being mounted.

As you can see, we can simply create a function at the global level and call it from our DeeplyNestedChild and pass the value for the state variable to be changed in the Parent component. Once called, we can set the state of the Parent component using its reference returned by ReactDOM.render().

This method can majorly help in cases, where we want to update the state of a Parent component in one component tree, from a Child component of some other component tree, considering if one of the component trees is getting dynamically generated using, say for example a Portal.

Child from Component Tree 2 updates the state of the topmost Parent of Component Tree 1

Only want to trigger a re-render? You can do so using forceUpdate() -