How react can Identify it's a class component or functional component?

Question

I have created two component in my React App one is class component and another one is functional component. But how react can identify when it call the component it's a class component or functional component?

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Answer 1

But how react can identify when it calls the component it's a class component or functional component?

// Who is a class component and who is a functional component?
ReactDOM.render(
  <>
    <ComponentA />
    <ComponentB />
    {React.createElement(ComponentA)}
    {React.createElement(ComponentB)}
  </>,
  document.getElementById('root')
);

While JSX represents objects , as stated in docs they are equivalent:

The above two components are equivalent from React's point of view.

But actually , React checking isReactComponent flag to determinate if it's a class component or function component:

// @ ReactComponent.js
ReactComponent.prototype.isReactComponent = {};

// @ ReactFiber.js
function shouldConstruct(Component: Function) {
  const prototype = Component.prototype;
  return !!(prototype && prototype.isReactComponent);
}

For more in-depth explanation check Dan Abramov's blog post .

Answer 2

React check the component for the isReactComponent flag (which is present on React.Component.prototype source ) to determine weather it's a class or a function.

You can read about it in detail here: https://overreacted.io/how-does-react-tell-a-class-from-a-function/