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Document.write("Message") It's the correct format

Either use (document.write) or use web technology like php
and pass the value through url and get on that page.

This keyword refers to the current object.

The this keyword is a synonym for the current object and can
be used with properties

‘setState’ function is used to update the state of a react component. If the UI of the component is dependent on the value we are updating, it will re-render the UI. For example :

setState({message : 'Hello'})

<strong><em> Easy learning curve :</em></strong>

React is easy to learn. Anyone with a good understanding of HTML, CSS and Javascript can learn to react easily.

<strong><em> Readability :</em></strong>

React uses JSX that improves the readability of the code.

<strong><em> Performance :</em></strong>

React uses virtual DOM to update the UI that doesn’t require to update the whole webpage for every single change. That makes the application faster.

<strong><em> Reusable components :</em></strong>

We can use one component in many places without rewriting the code.

<strong><em> SSR :</em></strong>

React supports server-side rendering that improves the page load time.

Keys are used to provide each list element with a stable identity. The keys should be unique. The best practice is to use ‘id’ of the data as the key.

React components are the basic building block of a react application. These are like functions that take inputs and returns one react element based on the inputs. Using the components, we can split an application into reusable, independent pieces.

When you use set State(), then apart from assigning to the object state React also re renders the component and all its children. We would get error like this: Can only update a mounted or mounting component. Hence we need to use this. stateto initialize variables inside constructor.