I use VisualStudio 2019 with TypeScript3.9 and Libman.
And I need Bootstrap4 and jQuery. So try get these libraries and typings(index.d.ts) by Libman.
Then Bootstrap4 typing(index.d.ts) get error "Cannot find module popper.js".
// Type definitions for Bootstrap 4.5
// Project: https://github.com/twbs/bootstrap/, https://getbootstrap.com
// Definitions by: denisname <https://github.com/denisname>
//                 Piotr Błażejewicz <https://github.com/peterblazejewicz>
// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
// TypeScript Version: 2.3
/// <reference types="jquery"/>
import * as Popper from "popper.js";     // ERROR!: "Cannot find module popper.js"
I found similar problems and answers.
- Bootstrap 4 TypeScript Type Definition fails to compile - Cannot find module 'popper.js'
- Cannot find module "popper.js" Angular 5 Visual Studio 2017 asp.net core
- Angular @types/bootstrap error: Namespace 'popper.js' has no exported member
It could only solve the problem temporarily.
I changed bootstrap4 typing(index.d.ts) to relative path as import * as Popper from "../popper_v1"; is fixed error.
But Libman override this manually-changed index.d.ts.
_
What can I do to fix this error? Do I need to install manually modified index.d.ts, like Popper v1.X?
Thanks.
Steps to reporduce error.
- Create ASP.Net4 MVC Project on .Net Framework 4.8 
- Create libman.json to add some libraries and TS typings 
{
  "version": "1.0",
  "defaultProvider": "jsDelivr",
  "libraries": [
    {
      "library": "bootstrap@4.5.2",
      "destination": "lib/bootstrap/"
    },
    {
      "library": "@types/bootstrap@4.5.0",
      "destination": "lib/types/bootstrap/"
    },
    {
      "library": "jquery@3.5.1",
      "destination": "lib/jquery/",
      "files": [
        "dist/jquery.js",
        "dist/jquery.min.js",
        "dist/jquery.min.map",
        "dist/jquery.slim.js",
        "dist/jquery.slim.min.js",
        "dist/jquery.slim.min.map",
        "external/sizzle/LICENSE.txt",
        "external/sizzle/dist/sizzle.js",
        "external/sizzle/dist/sizzle.min.js",
        "external/sizzle/dist/sizzle.min.map",
        "AUTHORS.txt",
        "LICENSE.txt",
        "README.md"
      ]
    },
    {
      "library": "@types/jquery@3.5.1",
      "destination": "lib/types/jquery/"
    },
    {
      "library": "@types/sizzle@2.3.2",
      "destination": "lib/types/sizzle/"
    },
    {
      "provider": "cdnjs",
      "library": "popper.js@1.16.1",
      "destination": "lib/popper.js/"
    },
    {
      "provider": "filesystem",
      "library": "lib_ManualInstallSources/popper_v1/",
      "destination": "lib/types/popper_v1/"
    }
  ]
}
** Note: I got typing(index.d.ts) of popper.js ver.1.X from GitHub repository.
- Create tsconfig.json
{
  "compilerOptions": {
    "noImplicitAny": false,
    "noEmitOnError": true,
    "removeComments": false,
    "sourceMap": true,
    "target": "es5",
    "lib": [ "ES6", "DOM" ],
    "baseUrl": ".",
    "typeRoots": [
      "./lib/types"
    ]
  },
  "exclude": [
    "node_modules",
    "wwwroot",
    "lib"
  ]
}
After these steps, bootstrap4 typing(index.d.ts) get error "Cannot find module popper.js" on import * as Popper from "popper.js";.
