Importing from 'rxjs/operators'

webpack.config.js

// Require the version of Rx you want to use (_esm5 or _esm2015)
const rxPaths = require('rxjs/_esm5/path-mapping');

var config = {
  // Other config options...
  resolve: {
    extensions: [".ts", ".js"],
    modules: [path.resolve('./src'), 'node_modules'],
    // Use the "alias" key to resolve to an ESM distribution
    alias: rxPaths(path.resolve(__dirname, 'node_modules'))
  }
}