code revamped using promise from callback functions

callback2promise.js

const DataModel = require('./DataModel'); // let's say the target model is 'DataModel', it's based on mongoose.

return DataModel.findOne({ "conditionKey": "conditionValue" }) // read data from db
  .then((data) => { // here update the data
    data.a = "test";
    data.b = 1111;
    return processData(data); // do some processing with data, let's suppose it returns (processed) data again.
  })
  .then((data) => {
     return data.save();
  })
  .then((data) => {
    console.log("finished");
  })
  .catch((err) => {
    console.log(err); // here catch all erros from the above steps.
  });