underscore package defines the
_ namespace on both the client
and the server.
We have slightly modified the way Underscore differentiates between objects and arrays in collection functions. The original Underscore logic is to treat any object with a numeric
lengthproperty as an array (which helps it work properly on
NodeLists). In Meteor’s version of Underscore, objects with a numeric
lengthproperty are treated as objects if they have no prototype (specifically, if
x.constructor === Object.