for is synchronous, so you will need to require the async library. That library has an async.each() function which is an asynchronous version of forEach.
You will want to read up on it here.
https://github.com/caolan/async#each
var async = require.main.require('async');
function doLast(){
console.log(" STRING ");
}
var array = [a, b, c];
async.each(array, function(value, next){
// The value is sent here, not the index, if you really need the index, you can use forEachOf() or do array.indexOf(value).
db.getObject(------,function(err,returnData){
//in this method modify string in some way
console.log("METHOD");
// Call next() to let the the app know that the async operation is complete.
next();
});
// When all async operations are complete, run doLast.
}, doLast);
