Not all browsers support the latest features of ES2015. The Babel project offers a polyfill that can be included into your TiddlyWiki so those features can be available to your plugins. To do this you will need a copy of the polyfill source.

You can obtain the source either through npm or downloaded/saved. See the Babel Polyfill documentation for specific information on installing it.

In your TiddlyWiki editions folder make sure you have a plugins/babel-polyfill folder. Then create the plugins/babel-polyfill/plugin.info file with the following in it:

{
  "title": "$:/plugins/babel/babel-polyfill",
  "description": "Babel Polyfills for ES2015 support",
  "author": "Your Name Here",
  "core-version": ">=5.0.0"
}

Create the folder plugins/babel-polyfill/files folder. Then create the plugins/babel-polyfill/files/tiddlywiki.files file with the following in it:

{
  "tiddlers": [
    {
      "file": "polyfill.min.js",
      "fields": {
        "title": "$:/plugins/babel/babel-polyfill/polyfill.min.js",
        "type": "application/javascript",
        "module-type": "library",
        "global-module": "true"
      }
    }
  ]
}

Now copy the polyfill.min.js you downloaded/saved.

Lastly you need a initializer so create the plugins/babel-polyfill/plugin.js file with the following in it:

/*\
title: $:/plugins/babel/babel-polyfill/plugin.js
type: application/javascript
module-type: startup

Load the babel-polyfill library on startup

\*/

exports.startup = function() {
  $tw.modules.execute('$:/plugins/babel/babel-polyfill/polyfill.min.js');
}

Now all the runtime ES2015 features are available like using Promise in your plugin code.

See Using ES2016 for Writing Plugins on how to use the ES2015 syntax for your plugin code.