在Web组件中使用外部JS库

Question

I am developing a web component using Polymer 2, and would like to make use of a third-party JavaScript library, which was not specifically designed for use with web components. As far as I know, the only way to do this is to include a <script> tag referencing the library, within the HTML file of my web component.

I can see a couple of issues with doing this, and want to know if there are any ways around them, and indeed whether including third-party libraries in this way is considered bad-practice.

1. The external library might set global variables which are visible to other components on the page, allowing web components to break each other, or break the page they are hosted on. Since encapsulation is often touted as one of the big advantages of using web components , this seems like a problem.

2. The external library might perform DOM queries or updates which would not be able to access the shadow-dom of the web component that is using them, so the external library might not actually work at all, or might update the hosting page's DOM again breaking encapsulation.

So, am I missing something or does this mean that including external libraries in a web component is a really bad idea? If so, it seems like a huge limitation of this technology, since we can't take advantage of the vast number of pre-existing JS libraries out there.

Answer 1

If you have an external library that does things like document.querySelector then you have two choices.

1. Choose to not use ShadowDOM with any of your components. If that is not an option, or if you really, REALLY want to use shadowDOM then:
2. You need to modify the third party library to allow a root element to be specified instead of always using document .

Beyond those two options you will probably not be able to use a third party library that assumes document will work for everything.

I guess the other option is to re-evaluate the third party library and see if it is REALLY worth using.

On my team we don't use third party libraries that are not just solid logic. Things like moment.js are just logic and we can use them without problems.

But something like jQuery? Yuck! I can't see needing something like that for a component.

Good luck!

Answer 2

I actually had to deal with that exact same issue yesterday so good timing ;) In my case the view in the first page has two sections one with radio buttons and because of business requirements depending on the user's radio button selection an input text with google maps autocomplete would get enabled (or stay disabled)

In this scenario it was much more efficient to load the page without the google maps libraries and then dynamically load the gmaps code after the webcomponent was fully rendered which lead to a 50% drop in time to interactive :) Here is what I ended up doing.

NOTE: The loadGoogleMaps() method and the initCalled variable declaration are outside the class and thus outside of the webcomponent (I put them under the import statements). I also omitted most of the class code from the example as it wasn't relevant to your question :)

import { html } from '@polymer/lit-element';
import { PageViewElement } from './page-view-element.js';
import { SharedStyles } from './shared-styles.js';
import '@vaadin/vaadin-radio-button/vaadin-radio-button.js';
import '@vaadin/vaadin-radio-button/vaadin-radio-group.js';
import { spinner } from './my-icons.js';

let initCalled;

function loadGoogleMaps() {
  //Only load gmaps if it has not been loaded before (tracked by the initCalled flag)
  if (!initCalled) {
    //Dynamically import the library and append it to the document header
    const script = document.createElement('script');
    script.type = 'text/javascript';
    script.async = true;
    script.onload = function () {
      //Code to execute after the library has been downloaded parsed and processed by the browser starts here :)
      initCalled = true;

      //TODO: Refactor this DOM traversing logic
      const searchAutocomplete = document.querySelector('my-app').shadowRoot.querySelector("home-property-view")
        .shadowRoot.querySelector('home-property').shadowRoot.querySelector("input[type='text']");

      const autocomplete = new google.maps.places.Autocomplete(
        searchAutocomplete, {
          types: ['address'],
          componentRestrictions: {  //Limit to only US addresses
            'country': 'us'
          }
        });

      autocomplete.addListener('place_changed', function () {
        const place = autocomplete.getPlace();
        dispatchEvent(new CustomEvent('propertyAddressChanged', {
          bubbles: true,
          composed: true,
          detail: place
        }));
      });
    };
    //Specify the location of the gmaps library
    script.src = '//maps.googleapis.com/maps/api/js?v=3.33&key=<YOUR-API-KEY-GOES-HERE>&libraries=places';

    //Append it to the document header
    document.head.appendChild(script);
  }
}

class HomeProperty extends PageViewElement {
  //....omitted class code for brevity...

  _didRender(props, changedProps, prevProps) {
    loadGoogleMaps();
  }

  //....omitted class code for brevity...
}