function getObjectDataByKey( field, theObject ){
	// get data
	return field.split('.').reduce((o,i)=> o[i], theObject);
}

// a simple object
const myObject = {
  name: 'John',
  lastname: 'Doe',
  data: {
    birthDate: '01/01/1970',
    placeOfBirth: 'San Francisco'
  }
}

// variable representing a key of the object
const field = 'data.birthDate';

console.log ( getObjectDataByKey ( field , myObject ) );
//will print
//01/01/1970

// also simple key will return correct value
console.log ( getObjectDataByKey ( 'name' , myObject) );
//will print
//John