quoteRegExp() - This is an example of overloading a function and annotating it with JSDoc.

quoteRegExp.js

/**
 * @type {{
 *   (input: string): string;
 *   (input: string, returnRegExp: true): RegExp;
 *   (input: string, flags: string): RegExp;
 * }}
 */
const quoteRegExp = function(input, flags) {
  input = input.replace(/[/\-\\^$*+?.()|[\]{}]/g, '\\$&');
  if (flags === true) flags = '';
  return 'string' === typeof flags ? new RegExp(input, flags) : input;
};

You can also do the following but the names will not carry over (nor will any associated comments):

/**
 * @type {quoteRegExp_1 & quoteRegExp_2 & quoteRegExp_3}
 */
const quoteRegExp = function(input, flags) {
  input = input.replace(/[/\-\\^$*+?.()|[\]{}]/g, '\\$&');
  if (flags === true) flags = '';
  return 'string' === typeof flags ? new RegExp(input, flags) : input;
};
/**
 * @callback quoteRegExp_1
 * @param {string} input
 * @param {?false=} flags
 * @returns {string}
 */
/**
 * @callback quoteRegExp_2
 * @param {string} input
 * @param {true} flags
 * @returns {RegExp}
 */
/**
 * @callback quoteRegExp_3
 * @param {string} input
 * @param {string} flags
 * @returns {RegExp}
 */

const a = quoteRegExp('asdf');            // VS Code type hint: string
const b = quoteRegExp('asdf', undefined); // VS Code type hint: string
const c = quoteRegExp('asdf', null);      // VS Code type hint: string
const d = quoteRegExp('asdf', false);     // VS Code type hint: string
const e = quoteRegExp('asdf', true);      // VS Code type hint: RegExp
const f = quoteRegExp('asdf', 'g');       // VS Code type hint: RegExp