I have to use mapReduce for a project and I started to follow the documentation.

I've created a test project following the first example from the page.

I've created a database named test in Mongo, and I inserted the object from example in collection col_one:

{
     _id: ObjectId("50a8240b927d5d8b5891743c"),
     cust_id: "abc123",
     ord_date: new Date("Oct 04, 2012"),
     status: 'A',
     price: 250,
     items: [ { sku: "mmm", qty: 5, price: 2.5 },
              { sku: "nnn", qty: 5, price: 2.5 } ]
}

My code is simple (like in example):

// MongoDB part
// Create server

var mapFunction1 = function() {
   emit(this.cust_id, this.price);
};

var reduceFunction1 = function(keyCustId, valuesPrices) {
   return Array.sum(valuesPrices);
};

collection.mapReduce(
   mapFunction1,
   reduceFunction1,
   { out: "col_two" }
);

// Print items from col_two

This throws this error:

.../node_modules/mongodb/lib/mongodb/connection/server.js:524
        throw err;
              ^ TypeError: undefined is not a function

If I change to this, this error disappears.

collection.mapReduce(
   mapFunction1,
   reduceFunction1,
   { out: "col_two" },
   function() {
       // Print items from col_two
   }
);

Why does the error disappear?

What you're hitting is one of the key differences between the API used in the shell and the native node.js driver: the shell is synchronous while the node.js driver is asynchronous.

Because the node.js driver is asynchronous, you must supply a callback parameter to the mapReduce call as indicated in the documentation so that you can receive the result.