Thanks for that clear and short answer Have a god day Sir Yoshi I think all will be ok with your help.

I get the number of harvester in each of my room with the code :

    let creepsInRoom = spawn.room.find(FIND_CREEPS);
    ....
    var numberOfHarvesters = _.sum(creepsInRoom, (c) => c.memory.role == 'harvester');

But each time an enemy come to one of my room and doesn't have a creep.memory.role attribute, I got a NPE. I can't find a way to check if an attribute exist in memory before trying to access it.

(I'm a JS newbie)