DBCollection.prototype._genIndexName generates index name automatically if it is not supplied.
It concatenates keys and direction. For example,

{field1: 1, field2: -1, field3: 1}

results with field1_1_field2_-1_field3_1
If instead of direction with have a string (2d) we might have duplicate "" or unneeded "" at the end.
Example 1:

{field1: 1, field2: "2d"}

"field1_1_field2_"

Example2:

{field1: 1, field2: "2d", field3: -1}

"field1_1_field2_field3-1"

It's not a big deal but other drivers might implement it in a different way
and the duplicated "_" might cause a confusion.

https://github.com/mongodb/mongo/blob/master/shell/collection.js#L218

Fix:
DBCollection.prototype._genIndexName = function( keys ){
var name = "";
for ( var k in keys )

{ var v = keys[k]; if ( typeof v == "function" ) continue; if ( name.length > 0 ) name += "_"; //name += k + "_"; // ------------- if ( typeof v == "number" ) //name += v; // --------------- name += "_" + v; // +++++++++ }

return name;
}