SPFX how to link the external libraries:

After create an sample spfx webpart with "nojavascript"

1. In visual studio code, open config.json file in your machine (press ctrl+p and type config.json file).

2. external object key shows empty initially.

3. Open Node JS cmd prompt and install requried external library files using npm

To Install Jquery: npm install jquery@2
Install Jquery types for typescript handy: npm install @types/jquery@2

Jquery UI

npm install jqueryui

npm install @types/jqueryui

Bootstrap

npm install bootstrap

Angular:

npm install @types/angular --save-dev

Under externals key object provide below reference

"externals": {

    "jquery":"node_modules/jquery/dist/jquery.min.js",

    "angular": {

      "path": "https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.6.6/angular.min.js",

      "globalName": "angular"

    },

    "bootstrap": {

      "path": "https://maxcdn.bootstrapcdn.com/bootstrap/3.4.0/js/bootstrap.min.js",

      "globalName": "bootstrap",

      "globalDependencies": ["jquery"]

    }

  },

In your webpart ts file, refer the files as below

import { SPComponentLoader } from '@microsoft/sp-loader';

let cssURL = "https://mercedessaga.sharepoint.com/sites/myfirstappcatalog/SiteAssets/bootstrap.min.css";

SPComponentLoader.loadCss(cssURL);

let cssURLIcon = "https://mercedessaga.sharepoint.com/sites/teja_dev/SiteAssets/CSS/font-awesome.css";

SPComponentLoader.loadCss(cssURLIcon);

SPComponentLoader.loadScript('https://cdnjs.cloudflare.com/ajax/libs/jquery/3.1.1/jquery.min.js', { globalExportsName: 'jQuery' }).then((jQuery: any): void => {

SPComponentLoader.loadScript('https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.7/js/bootstrap.min.js',  { globalExportsName: 'jQuery' }).then((): void => {        

});