怎样将ES6 bare module引入到本地程序中，3DTilesRendererJS集成问题解决方案探索

最近想要将NASA的3DTile的解析渲染代码集成到自己的程序中，在github中clone了

git+https://github.com/NASA-AMMOS/3DTilesRendererJS.git 相关代码

在运行其示范代码时却出现以下问题：

“Uncaught SyntaxError: Cannot use import statement outside a module”

因为引用文件引入了class，这是ES6中module中定义的概念，需要在script中加入 type=“module”才能确保浏览器正确解析脚本，如下：

 

虽然浏览器可以解析脚本了，紧接着运行又出现以下报错

“Uncaught SyntaxError: Cannot use import statement outside a module” when importing ECMAScript 6”

查阅资料才发现原因：

从第三方导入ES6模块时，仅使用浏览器同步是行不通的。

因为采用import导入相关模块时采用了bare import，如：

import { LineSegments, BufferGeometry, Vector3, BufferAttribute, LineBasicMaterial } from 'three';

如：上面导入的‘three’，

 bare import是使用像Webpack这样的捆绑器时通常会看到的导入，它不是指向node_modules的相对路径，而是映射到了打包发布的路径，例如 通过Webpack打包发布后是可以正常工作的， 但是，如果直接在浏览器中运行，则会看到：

Uncaught TypeError: Failed to resolve module specifier "three". Relative references must start with either "/", "./", or "../".

解决方案：

1）直接引用在线库，某些库都有在线打包发布的地址：

import * from "https://code.jquery.com/jquery-3.1.1.min.js"

2）用Webpack进行打包，这个比较麻烦

1. `npm install --save-dev webpack webpack-cli copy-webpack-plugin webpack-dev-server `

2. Create webpack.config.js:

   const path = require('path');
   const CopyPlugin = require('copy-webpack-plugin');
   
   module.exports = {
       entry: './src/app.js',
       mode: 'development',
       output: {
           path: path.resolve(__dirname, 'dist'),
           filename: 'app.js'
       },
       devServer: {
           contentBase: './dist'
       },
       plugins: [
           new CopyPlugin([
               { from: 'src/index.html', to: './' },
               { from: 'src/style.css', to: './' },
           ]),
       ],
   };

3. Add a script to the package.json: "dev": "webpack-dev-server --open"

4. The import can now look like this:

   import { html, LitElement } from 'lit-element/lit-element.js';

Run the dev-server with live-reload (similar to browser-sync) with npm run dev.

After trying it for a small application and really only doing the bare minimum with Webpack, I have to say it is a viable option. But it requires to download some dependencies from NPM and create a webpack.config.js.

3）Open Web Components (OWC)

Open Web Components offer a simple dev-server that does nothing more than rewrite the bar module imports to relative imports.

npm install --save-dev owc-dev-server

After trying it out, I was disappointed to find that the dev-server does not offer live-reloading.

The best solution I found was to combine it with browser-sync. Here are the scripts I added to my package.json

"dev": "owc-dev-server | npm run watch",
"watch": "browser-sync start -f src/ --proxy localhost:8080 --startPath src",

Note that watch is just a helper-script used by dev; so you have to use npm run dev.

4）Polymer-cli

The last tool I tried was Polymer-CLI. In the end, the approach is a mix between the previous two. It requires an additional polymer.json config-file and it also does not function without browser-sync.

The steps are:

1. npm install --save-dev polymer-cli

2. Create polymer.json:

   {
       "entrypoint": "src/index.html",
       "shell": "src/app.js",
       "npm": true
   }

3. Set up scripts:

   "watch": "browser-sync start -f src/ --proxy localhost:8000 --startPath src",
   "dev": "polymer serve --open-path src/index.html | npm run watch"

以上这些方法可参考：http://dplatz.de/blog/2019/es6-bare-imports.html