my-screeps/dist/role.harvester.js

var roleHarvester = {

  /** @param {Creep} creep **/
  run: function(creep) {
    if(creep.carry.energy < creep.carryCapacity) {
      var sources = creep.room.find(FIND_SOURCES);
      if(creep.harvest(sources[0]) == ERR_NOT_IN_RANGE) {
        creep.moveTo(sources[0]);
      }
    }
    else {
      var targets = creep.room.find(FIND_STRUCTURES, {
        filter: (structure) => {
          return (structure.structureType == STRUCTURE_EXTENSION ||
                  structure.structureType == STRUCTURE_SPAWN ||
                  structure.structureType == STRUCTURE_TOWER) && structure.energy < structure.energyCapacity;
        }
      });
      if(targets.length > 0) {
        if(creep.transfer(targets[0], RESOURCE_ENERGY) == ERR_NOT_IN_RANGE) {
          creep.moveTo(targets[0]);
        }
      }
    }
  }
};

module.exports = roleHarvester;