Support JavaScript intellisense inside HTML

Sorry, I can't open a question in stackoverflow.com because it has a js error.

Help:

1. When I set the editor's lang to html, the javascript IntelliSense is not available. How to use both html and javascript IntelliSense in one editor.
2. When I use import in es6, how to use js IntelliSense from module in another file?

import {List} from "./List"
List.of
// there is no js IntelliSense

List.js

export class List {
  constructor(head, tail) {
    this.head = head
    this.tail = tail
  }

  static of(...elements) {
    let result = null
    for (let i = elements.length - 1; i >= 0; i--)
      result = new List(elements[i], result)
    return result
  }

  map(f) {
    return new List(f(this.head), this.tail && this.tail.map(f))
  }

  *[Symbol.iterator]() {
    for (let pos = this; pos; pos = pos.tail)
      yield pos.head
  }
}

Start with the issue's HTML editor configuration and the JavaScript examples using import, export, and List.of. Determine how HTML and JavaScript IntelliSense are expected to coexist and how cross-file module completion should behave; done means both requested IntelliSense scenarios work in one editor.