My web part needed to read a querystring. SharePoint Framework has a tool for that!
When I have created little chunks of JavaScript in the past and wanted to read the querystring, I needed to write my own code to get to the various parts easily. Fortunately there is a way to do this thanks to a useful blog post by Paul Matthews:
var querystring = new UrlQueryParameterCollection( window.location.href ); var id = querystring.getValue( "myId" );
This handy class lives in the Microsoft SP Core library, remember to add an import for it in your script:
import { UrlQueryParameterCollection } from '@microsoft/sp-core-library';