argnames.js

const functionMatch = /^function\s*\w*\(([\w\s,]+)\) {/;

/**
 * @param {!Function} fn
 * @return {!Array<string>} array of simple arg names (no =, ... etc)
 */
function simpleArgNames(fn) {
  const match = functionRe.exec(fn.toString());
  if (!match) { return []; }
  const args = match[1].split(',').map((x) => x.trim());
  if (args.length && args[args.length - 1] === '') {
    args.pop();
  }
  return args;
}