// This is our method for doing optional parameters in javascript.
// In this example source is a mandatory parameter, opts are optional,
// and callback is an optional callback. Optional parameters are stored
// in the object opts.
const myfunction = (source, opts, callback) => {
  // check if any optional parameters were provided by looking at the type of the second argument
  if (typeof opts === 'function') {
    // If opts is a function it's the callback
    callback = opts;
    // Set opts to be an empty object
    opts = {};
  }
  // Then create consts for all parameters, containing
  // either the value in opts, or the default value.
  // So this is where you providde default values.
  const params = opts && opts.params ? opts.params : {};
  const options = opts && opts.options ? opts.options : {};
  const commitHash = opts && opts.commitHash ? opts.commitHash : 'master';

  // do stuff

  if (callback) callback(null);
};