Babel reload script concept

babel-node.mjs

/**
 * This tiny wrapper file checks for known node flags and appends them
 * when found, before invoking the "real" _babel-node(1) executable.
 */

import getV8Flags from "v8flags";
import path from "path";

getV8Flags(function(err, v8Flags) {
  if (err != null) throw err;
  const babelPath = path.join(__dirname, "_babel-node");
  let args = [];
  let execArgv = [];
  let native = new Set([
    ...v8Flags,
    "debug", "--debug", "--debug-brk",
    "--inspect", "--inspect-brk",
    "--expose-gc",
  ]);
  let aliases = new Map([
    ["-d", "--debug"],
    ["-gc", "--expose-gc"],
    ["-r", "--require"],
  ]);

  for (let i = 2; i < process.argv.length; i++) {
    let arg = process.argv[i];

    if (arg === "--") {
      while (i < process.argv.length) args.push(process.argv[i++]);
      break;
    }

    const delimIndex = arg.indexOf("=")
    const flag = delimIndex < 0 ? arg : arg.slice(0, delimIndex);
    const resolved = aliases.get(flag) || flag;
    
    if (resolved === "--require") {
      execArgv.push(resolved);
      if (i + 1 < process.argv.length) {
        let value = process.argv[i + 1]
        if (value !== "--") {
          execArgv.push(value);
          i++;
        }
      }
    } else {
      /**
       * Replace dashes with underscores in the v8Flag name
       * Also ensure that if the arg contains a value (e.g. --arg=true)
       * that only the flag is returned.
       */
      if (resolved.includes("--")) {
        resolved = `--${resolved.slice(2).replace(/-/g, "_")}`;
      }
      if (native.includes(resolved)) {
        execArgv.push(resolved);
      } else {
        args.push(arg);
      }
    }
  }

  require("child_process").replaceFork(babelPath, args, {execArgv});
});