Q: Adding multiple JS files into Angular? i need to know how can I import all these js files inside Angular globally so that every component can use it.
<script src="assets/js/vendors/modernizr-3.7.1.min.js"></script>
<script src="assets/js/vendors/jquery-3.3.1.min.js"></script>
<script src="assets/js/vendors/bootstrap.bundle.min.js"></script>
<script src="assets/js/vendors/jquery.meanmenu.min.js"></script>
<script src="assets/js/vendors/slick.min.js"></script>
<script src="assets/js/vendors/jquery.nice-select.js"></script>
<script src="assets/js/vendors/jquery.countdown.min.js"></script>
<script src="assets/js/vendors/jquery.ajaxchimp.min.js"></script>
<script src="assets/js/vendors/jquery.fancybox.min.js"></script>
<script src="assets/js/vendors/imagesloaded.pkgd.min.js"></script>
<script src="assets/js/vendors/isotope.pkgd.min.js"></script>
<script src="assets/js/vendors/instafeed.min.js"></script>
<script src="assets/js/vendors/jquery.nstslider.min.js"></script>
<script src="assets/js/vendors/ScrollMagic.min.js"></script>
<script src="assets/js/main.js"></script>

I tried adding this inside index.html like you would normally do without but i didn't work.

A: you should  load the Javascript in the order that it's needed. If you're using some jQuery plugins, then you should load jQuery before those plugins, as they may instantiate something that uses the jQuery object(s) without you knowing.

A: You have to install them using
npm install
For example npm install bootstrap
Add them in angular.json file like that
"build": {
          "builder": "@angular-devkit/build-angular:browser",
          "options": {
            "outputPath": "dist/RecipeBook",
            "index": "src/index.html",
            "main": "src/main.ts",
            "polyfills": "src/polyfills.ts",
            "tsConfig": "tsconfig.app.json",
            "aot": false,
            "assets": [
              "src/favicon.ico",
              "src/assets"
            ],
            "styles": [
              "node_modules/bootstrap/dist/css/bootstrap.min.css",
              "src/styles.css"
            ],
            "scripts": [
              "node_modules/bootstrap/dist/js/bootstrap.min.js"
            ]
          }

