React in a Legacy App

I am working on a relatively old legacy web app. It’s a very complex system, and was built upon jQuery and uses requirejs for module loading. I have been slowly updating it to use ES5+ features and React. In particular, working with React in this app has been a bit painful. This app, you see, doesn’t have a build environment, so using webpack and babel aren’t available, at least not in the standard way. With a great deal of effort, it certainly can be made to work with a build environment, but my intention is to spend my energy on cleaning up the code and, when that task is complete, bringing in a standard build system. Ah, that will be great.

But, at least for today, I want to add some React code. I’ve done it before in this app, but since I have no build system, I’ve used React.createElement(). If you’ve tried to build anything more than a trivial Component using React.createElement(), you know that it gets old, fast.

So I want to just JSX, naturally. But with no build system, I wasn’t sure how to proceed. There are plenty of examples on how to use babel standalone and mark your script tags as "text/babel" to have your JSX code converted to JS on the fly. But, a couple problems. First, I’m using requirejs, and it doesn’t want to mark only some script tags as "text/babel" (it can mark all or none). I’ve even gone so far as modifying requirejs to only mark certain script tags as "text/babel" but that leads to the next problem: babel standalone doesn’t seem to compile the scripts imported by requirejs. I’m sure there’s a way, but I gave up on that approach.

What I really wanted was something that would just compile JSX files into JS, so I could write complex React Components and not worry about it.

I found some examples for using browserify and babel to basically do what a build system would do, but it ends up creating massive JS files with all kinds of transpiled code. Clearly I was doing this wrong.

I finally found that I can use a basic babel transformFile() call with some relatively simple configuration that mainly just compiles the React JSX into React JS with React.createElement() calls, and really no other change to my code.

I wrote a “watcher” script for this purpose, and now just keep it running in my IDE while I work:

 const args = require('minimist')(process.argv.slice(2));
 const fs = require("fs/promises");
 const path = require("path");
 const chokidar = require("chokidar");
 const babel = require("@babel/core");
 const options = {
   ignored: /(^|[\/\\])\../
 async function compile(src) {
   return new Promise((resolve, reject) => {
     const info = path.parse(src);
     const dest = path.join(info.dir, `${}.js`);
     babel.transformFile(src, {}, async (error, result) => {
       if (error) { return reject(error) }
       await fs.writeFile(dest, result.code);
       console.log(`compiled ${src} -> ${dest}`);
 async function remove(src) {
   const info = path.parse(src);
   const dest = path.join(info.dir, `${}.js`);
     .then(() => {
       console.log(`removed ${dest}`);
     .catch(error => {
       // ignore
 (async () => {'**/*.jsx', options).on('all', async (event, path) => {
     if (event === "add" || event === "change") {
       await compile(path);
     if (event === "unlink" && args.delete) {
       await remove(path);

The babel configuration file (babel.config.json) looks like this:

   "presets": [
         "targets": {
           "edge": "17",
           "firefox": "60",
           "chrome": "67",
           "safari": "11.1"
         "useBuiltIns": "usage",
         "corejs": "3.6.5"
   "plugins": ["@babel/plugin-syntax-class-properties"]

For packages, I’ve installed these in my package.json file:

chokidar minimist 

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