app.LoadA( function cba() {
        app.LoadB( function cbb() {
            ....
            app.LoadC( function cbc() {
            ...
                app.LoadD( function cbd() {
                    ...
                } );
            } );
        } );
    } );

LoadA( cb )
{
    let url = '.........'; // just an url
    let req = new XMLHttpRequest();
    req.open('GET', url, true);
    req.responseType = 'arraybuffer';

    let lh = this;

    req.onload = function ol(event)
    {
        let arrayBuffer = req.response;
        let loadedObject = Convert( arrayBuffer, ......... );
        cb( loadedObject ); // call, if successed!
    }
    req.send(null);
}
....

loadA().then(function() {
  loadB().then(function() {
    loadC().then(...
  });
});

loadA().then(function() {
  return loadB();
).then(...)

loadA().then(loadB).then(loadC).then(...)

function wrapInsidePromise(f) {
  return new Promise(function(resolve, reject) {
    f(function() {
      resolve();
    });
  });
}

var pLoadA = wrapInsidePromise(app.loadA);
var pLoadB = wrapInsidePromise(app.loadB);
...

pLoadA().then(pLoadB).then(...);

async function() {
  await loadA();
  await loadB();
  ...
  let finalResult = await loadN();
  ...
}

app = {

    loadA: function() {
        return new Promise((resolve) => {
            console.log('Loading A');
            resolve();
        });
    },

    loadB: function () {
        return new Promise((resolve) => {
            console.log('Loading B');
            resolve();
        });
    },

    // For demonstration, I have coded loadC with a rejection if it fails
    // (Success is based on a random boolean)
    loadC: function () {
        return new Promise((resolve, reject) => {
            console.log('Loading C');
            var success = Math.random() >= 0.5;
            if( success ){
                resolve();
            } else {
                reject('C did not load');
            }

        });
    },

    loadD: function () {
        return new Promise((resolve) => {
            console.log('Loading D');
            resolve();
        });
    }

};

// A global function for reporting errors
function logError(error) {
    // Record the error somehow
    console.log(error);
}

app.loadA().then(app.loadB).then(app.loadC, logError).then(app.loadD);

app.loadA().then(app.loadD).then(app.loadB).then(app.loadC, logError);