function-name - Get the name of a named function

function-name.js

/*!
        function-name
        Get the name of a named function
        https://github.com/sindresorhus/function-name
        by Sindre Sorhus
        MIT License
*/
function functionName(fn) {
  if (typeof fn !== 'function') {
    throw new TypeError('Expected a function.');
  }

  return fn.name || (/function ([^\(]+)/.exec(fn.toString()) || [])[1] || null;
}