Converting ObjectId Values to year-month labels v2

A long time ago I wrote a blog post showing how to convert ObjectId value field to corresponding "YYYY-MM" string for reporting type applications.  Since I wrote that, aggregation pipeline gained the "$switch" expression which makes the syntax a lot shorter and easier to express (and read).

For variety, this version converts *string* type that represents ObjectId value to corresponding year-month:

d=[];
o=[];
for (yr=2011;  yr < 2018; yr++ ) {
       for (m=1; m<13; m++) {
           if (m<10) mo="0"+m; else mo=""+m;
           var dt=new ISODate(""+yr+"-"+mo+"-01T00:00:00Z");
           d.push(""+yr+"-"+mo);
           /* wrap string in 'new ObjectId()' to convert OID rather than string type */
           o.push(""+(dt.getTime()/1000).toString(16)+pad);
      }  
}
makeLabeledSwitch = function(field, keys, values) {
      var sw = {$switch:{
           "branches":[ ],
           default:"other"}
      };
      var br=[];
      var maxPos=keys.length;
      var first="<" + keys[0];
      br.push({case:{$lt:[field,values[0]]}, then:first})
      for (pos = 0; pos < maxPos-1; pos++) {
           br.push({case:{$lt:[field,values[pos+1]]}, then: keys[pos] });
      }
      var last=">" + keys[maxPos-1];
      sw["$switch"]["default"] = last;
      sw["$switch"]["branches"] = br;
      return sw;
}

This syntax is more straight forward, and what it does is quite similar, which is for every "YYYY-MM" string in the range you're interested in, it maps the range of ObjectId string values (or technically, its first 4-bytes) to the year-month range.   If you want to make this work with actual ObjectId type rather than string type, change the loop to populate "o" array with ObjectId() of corresponding string.