Import XML files to MongoDb with Nodejs
Variable declaration keywords
One thing I notice is that the var keyword is used in some functions while others use const. The variables dbo, MongoClient and url are never reassigned so const can be used for those. Using const when re-assignment isn't needed is a good habit because it can help avoid accidental re-assignment and other bugs. Many users also believe var should not be used with ES6 code - refer to the ES lint rule no-var.
nested callbacks
The code is bordering on callback hell. I'd suggest moving anonymous functions out to being named functions. This will also help with automated testing. Let's look at the inner most function:
const insertionCallback = (db, err, res) {
    if (err) throw err;

    db.close();
};

It needs a reference to db. That could be made a parameter:
function (db, err, res) {

Then when making a reference to that function, make a partially applied function with db as the first argument:
dbo.collection(&quot;entity&quot;).insertMany(result, insertionCallback.bind(null, db))

Or instead of accepting db as a parameter, set the context to it:
dbo.collection(&quot;entity&quot;).insertMany(result, insertionCallback.bind(db))

after doing that this could be used instead of db within that insertionCallback function.
Then continue pulling out those nested functions - e.g. :
const insertResult = (result, err, db) =&gt; {
    if (err) throw err;

    const dbo = db.db(&quot;myDb&quot;);

    dbo.collection(&quot;entity&quot;).insertMany(result, insertionCallback.bind(null, db));
};

Then that function can be used in its outer function:
const handleParsedStr = (err, result) =&gt; {

    const MongoClient = require('mongodb').MongoClient;
    const url = &quot;mongodb://localhost:27017/&quot;;

    MongoClient.connect(url, insertResult.bind(null, result));
});

And that functions can be used as the callback to parser.parseString():
parser.parseString(data, handleParsedStr);

error handling
Some of the callback functions check error arguments like err and conditionally handle such cases, but others don't- e.g. the callback to parser.parseString(). It is advisable to handle such errors, and would also make the code more consistent.
parser declared within forEach callback function
The variable parser is declared within the callback function to the .forEach() method. This means the parser is created once for each file in the directory. If possible, consider declaring it outside the loop to reduce resource requirements.