How to use a variable as a field name in mongodb-native findOne()?

Just put the variable in []

var name=req.params.name;
var value = req.params.value;
collection.findOne({[name]:value}, function(err, item) {
res.send(item);
});

You need to set the key of the query object dynamically:

var name = req.params.name;
var value = req.params.value;
var query = {};
query[name] = value;
collection.findOne(query, function (err, item) { ... });

When you do {name: value}, the key is the string 'name' and not the value of the variable name.

I'd like to clarify that if you're trying to make a query concerning a nested field only (not its value), like if you want to query the field "name" from this document:

{
    loc: [0, 3],
    unit: {
        name : "playername"
    }
}

this will work (as in my case - using update):

mdb.cords.updateOne(
    {_id: ObjectID(someid)}, 
    {$set: {[query]: newValue}}, 
    function (err, result) {
        ...
    }
}

Simply enclosing [query] in brackets tells mongodb that it's not literal, but rather a path.