Javascript function confused syntax

I am a little bit confused with the code below. It’s obviously a two arrow function written in es6 code but I do not understand exactly some parts.

1. The 2nd parameter named done is an empty function that does nothing? Or it is executed with a simple return as the result from the second arrow anonymous function?

2. the XXXX.load is a promise function that returns some results. How the caller of the Index can get the results of the 2nd parameter done ie done(null, result) ?

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3. What is the equivalent code in es5?

const Index = (name, done = () => {}) => (dispatch, getState) => {
  
  return XXXX.load()
    .then((result) => {
      dispatch({type:OK});
      done(null, result);
    })
    .catch((error) => {
      dispatch({type:ERROR});
      done(error);
    });
};

>Solution :

1. The empty function is a default value for done. Default values prevents runtime crashes.

2 and 3 can be understood by seeing below code: (simply run it and see the consoles.

const DEFAULT_FUNCTION_VALUE = ()=> {};

const XXXX = {
  load: function() {
    return new Promise(function(resolve, reject) {
      setTimeout(function() {
         resolve({data: 'from XXXX.load'});
      },2000);
    });
  }
}

const Index = function(name='', done=DEFAULT_FUNCTION_VALUE) {
  return function(dispatch, getState) {
    return XXXX.load().then(function(result) {
      console.log({result});
      done(result);
    }).catch(function(error) {
        console.log(error);
    });
  }
}

function doneImplementation(data) {
  console.log('Data from done- ', data);
}

Index('', doneImplementation)();