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1.0.0-pre.1 • Public • Published


Middleware for Koa servers that transforms standard JavaScript modules to earlier versions of JavaScript and/or AMD modules (inlining loader script @polymer/esm-amd-loader into the HTML), for use with older browsers.

Consider an HTML file containing the following inline JavaScript module:

<script type="module">
import {someFunction} from './some-module.js';

The koa-esm-to-amd middleware would transform that HTML code to something like the following:

// An inline loader script from `@polymer/esm-amd-loader` package
// which adds the `define` function used below.
define(["./some-module.js"], function (someModule) {

Because this is middleware, you can use it in a simple static file server as well with a proxy server. This makes it possible to use in front of a test server such as the one karma starts up. (See koa-karma-proxy for examples.)

Note: HTML and JavaScript are parsed on every request for those content-types, it is intended for use in development context to facilitate build-free testing/iteration as opposed to in a high volume production web server.

How it works

By default, a set of babel transform plugins are chosen based on the known capabilities of the browser/user-agent identified in the Koa Context for the request.

When the downstream server returns HTML content, it is scanned for module script tags (e.g. <script type="module">). Any src attribute values are appended with the __esmTransform query parameter. The query parameter is added because the middleware needs to distinguish between scripts that are being imported as a module vs a traditional script and this can't necessarily be determined by looking at the script itself.

Inline module script content and URLs with the __esmTransform query parameter have their import specifiers appended with the __esmTransform to indicate the requested content should be treated as a module and compiled with the selected babel plugins.

Depending on plugins being used, certain support scripts will also be inlined into the HTML, such as @polymer/esm-amd-loader and regenerator-runtime.


  • babelPlugins: Either an Array of babel plugins (e.g. [require('@babel/plugin-transform-modules-amd')]) or a Function that takes a Koa Context and returns an Array of babel plugins (ctx) => []. Providing a value for this option will override the default behavior of the middleware's capabilities-based babel plugin selection.
  • exclude: An array of requested paths or minimatch based patterns to match requested paths that should be excluded from any process/rewriting by the middleware.
  • queryParam: You can redefine the appended query parameter string from __esmTransform as something else.
  • logger: Middleware will call debug, info, warn and error methods on console to log events. If you use a different logger for your application, provide it here.
  • logLevel: Set a minimum level for events to be logged to override the default level of info.




npm i koa-esm-transform

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