How does React Lazy fetch components? - reactjs

Question

I recently read about React Lazy and how it "loads" the components during run time when they are required to be rendered. I assume that "loading" here means fetching the component from the server and then rendering it.
So my question is, how does React manage this fetching of components? How does it know the exact path from where to fetch this component (given that our code will mention the relative path but fetching will require complete server path)? Does it depend on Webpack for this?

Answer 1

Let's look into the React code. React.lazy is defined as follows.
export function lazy<T, R>(ctor: () => Thenable<T, R>): LazyComponent<T> {
let lazyType = {
$$typeof: REACT_LAZY_TYPE,
_ctor: ctor,
// React uses these fields to store the result.
_status: -1,
_result: null,
};
if (__DEV__) {
// ... additional code only in development mode
}
return lazyType;
}
As you can see, React.lazy requires a Promise which resolves to a module with a default export containing a React component (freely cited by React Docs). This also means that not React resolves the file, but import() does. import() works as documented in the MDN.
The async import() is a new function in ES6 which is not available in all browsers but can be polyfilled by Webpack and Babel/Typescript/others.
What you often see is code like the following, which automatically splits the imported file away by Webpack.
import(/* webpackChunkName: "xyz" */ './component/XYZ')
This creates a new javascript xyz.js next to your bundle script.
If you don't use Webpack, you need to create those files by yourself. Webpack just reduces the work required from you. So you don't absolutely depend on Webpack. This approach might look like the following:
// ./component/xyz.js
export default function() { return <div>Component</div> }
// ./main.js
const OtherComponent = React.lazy(() => import('./component/xyz.js'));
export default function() { return <div>Component</div> }
And the file structure:
| public
|---| main.js
|---| component
|---| --- | main.js
As you see, no webpack required. It just makes your life easier.