callback_to_promise.js

//normally calling an async function
get_value_from_DB(function(err, data){
    if(err)
        //handle error
    else
        //work with data
})

//converting async function to promise and calling it
new Promise(function(resolve, reject){
    get_value_from_DB(function(err, data){
        if(err) reject(err);
        else resolve(data);
    })
})