Switch from fnRender to mRender

I have always used fnRender to combine columns.

"aoColumnDefs": [
            { // merge address1 and 2 into this column
                "fnRender": function ( oObj ) {
                    return oObj.aData[2] +'<br>'+ oObj.aData[3];
                },
                "aTargets": [ 2 ]
            },
            { // hide address2 column
                "bVisible": false,
                "aTargets": [3]
            {
        ]

I am at a loss doing this with mData and mRender.

mRender: function ( data, type, row ) {
                         var formatted = ""
                         formatted = (data + '<br>' + data[2])
                         return formatted;}
                         },

Returns "Alexandra e". The field and the third character of the field.

Help, please. What simple thing am I not doing?