Use JavaScript library with Angular v4+

app.component.ts

import {Component} from '@angular/core';
import * as MobileDetect from '../../node_modules/mobile-detect/mobile-detect';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  md: any;
  mobile: any;

  constructor() {
    this.md = new MobileDetect(window.navigator.userAgent);
    this.mobile = (this.md.mobile() == null) ? 'is not' : 'is';
  }
}

app.component.html

<h1>
  Your device {{mobile}} mobile.
</h1>