Sharepoint - How to retrieve pageContext in SPFx?

this.context.pageContext is available on render() method within top level class that extends BaseClientSideWebPart. Simply pass this.context as a props of to HelloWorld.tsx to reuse it.

Sample (HelloWorldWebpart.ts):

public render(): void {
const element: React.ReactElement<any > = React.createElement(
   HelloWorld,
   {
     description: this.properties.description,
     context: this.context
   }
);

 ReactDom.render(element, this.domElement);
}

And in HelloWorld.tsx you should be able to retrieve the context as this.props.context

Or alternatively use react redux to store the context on initialization onInit() method (https://dev.office.com/sharepoint/reference/spfx/sp-webpart-base/oninit-baseclientsidewebpart)

It is better to use correct type WebPartContext instead of any. Don't forget to put namespace.

import { WebPartContext } from '@microsoft/sp-webpart-base';

export interface WebPartProps {
   context: WebPartContext;
   description: string;
}
...

public render(): void {
   const element: React.ReactElement<WebPartProps> = React.createElement(
   HelloWorld,
   {
     description: this.properties.description,
     context: this.context
   }
);

 ReactDom.render(element, this.domElement);
}

Here is more info WebPartContext class