Add Bootstrap 4 to Angular 6 or Angular 7 application

You need to do the following:

Add Bootstrap to package.json (which is done with npm install bootstrap --save)

Add Bootstrap to angular.json (jQuery is required too)

  "styles": [
     "styles.css".
     "../node_modules/bootstrap/dist/css/bootstrap.min.css"
  ],
  "scripts": [
     "../node_modules/jquery/dist/jquery.min.js",
     "../node_modules/popper.js/dist/umd/popper.min.js",
     "../node_modules/bootstrap/dist/js/bootstrap.min.js"
  ],

Reference Bootstrap in the Angular app (app.module.ts)

import bootstrap from "bootstrap";

Demo: https://codesandbox.io/s/kmm2q7zvko
Article: https://medium.com/wdstack/angular-7-bootstrap-4-starter-25573dff23f6

OPTION-1

execute

npm install bootstrap@4 jquery --save

The JavaScript parts of Bootstrap are dependent on jQuery. So you need the jQuery JavaScript library file too.

"styles": [
    "src/styles.css","node_modules/bootstrap/dist/css/bootstrap.min.css"
],
"scripts": [
    "node_modules/jquery/dist/jquery.min.js",
    "node_modules/bootstrap/dist/js/bootstrap.min.js"
]

OPTION-2

ng-bootstrap It contains a set of native Angular directives based on Bootstrap’s markup and CSS. As a result, it's not dependent on jQuery or Bootstrap’s JavaScript

npm install --save @ng-bootstrap/ng-bootstrap

After Installation import it in your root module and register it in @NgModule imports` array

import {NgbModule} from '@ng-bootstrap/ng-bootstrap';
@NgModule({
    declarations: [AppComponent, ...],
    imports: [NgbModule.forRoot(), ...],
    bootstrap: [AppComponent]
})

ng-bootstrap requires Bootstrap's 4 css to be added in your project. you need to Install it explicitly via:

npm install bootstrap@4  --save 

In your angular.json add the file paths to the styles array in under build target

"styles": [
    "src/styles.css",
    "node_modules/bootstrap/dist/css/bootstrap.min.css"
],