how to let MongoDB reconnect when it hanged or crushed in nodejs

I'm using nodejs and a mongoDB , and I'm having some connection issues. below is my code:

let url='mongodb://xxx';
let mongoClient = require('mongodb').MongoClient;
mongoClient.connect(url,(err, db)=>{
    routes(app, db);
    let server = http.Server(app);
    server.listen(port,(err)=>{})
})

sometime it seems crushed or hanged, so how to let it reconnect automatically?or is there exits a better solution?

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