I just want to point out that none of the roles were set at .length < 0 when I actually tried the code.

I am having a lot of trouble the game spawning less important creeps before harvesters which thereby results in my population dying out. I have tried to formulate a priority order in my code but it has no worked so far. Would someone be able to help me with what I am doing wrong? I tried nesting 'if' and 'else if' statements before my code but this did not work. I also tried:

let stage1 = _(Game.creeps).filter( { memory: { role: 'harvester' } } ).size(4);

let stage2 = _(Game.creeps).filter( { memory: { role: 'extender' } } ).size(2);

 if(stage1 === false) {
if(harvesters.length < 4) {
    var newName = 'harvester' + Game.time;
    console.log('Spawning new harvester: ' + newName);
    Game.spawns['Spawn1'].spawnCreep([WORK,WORK,CARRY,CARRY,CARRY,MOVE,MOVE,MOVE,MOVE,MOVE], 
 newName, 
        {memory: {role: 'harvester'}});
}}

 else if (stage1 === true) {   
if(extenders.length < 0) {
    var newName = 'extender' + Game.time;
    console.log('Spawning new extender: ' + newName);
    Game.spawns['Spawn1'].spawnCreep([CARRY,CARRY,CARRY,CARRY,MOVE,MOVE,MOVE,MOVE,MOVE], 
   newName, 
        {memory: {role: 'extender'}});
}}

if(stage1 & stage2 === true) {
        if(southharvesters.length < 0) {
    var newName = 'southharvester' + Game.time;
    console.log('Spawning new southharvester: ' + newName);
    Game.spawns['Spawn1'].spawnCreep([WORK,WORK,CARRY,CARRY,CARRY,MOVE,MOVE,MOVE,MOVE], newName, 
        {memory: {role: 'southharvester'}});        
}}   

This pattern I continued on through all my different creep roles in order of priority but it did not work although no error message came up. Is there a particular way in javascript which is best to create priority ques? Thanks for your help.

@famine Thanks for pointing me in the right direction, I used let. The code is a bit-long winded - I am surprised it worked tbh - but it did

run: function(creep) {
let prospect = new RoomPosition(34,4, 'W57S45');
let home = new RoomPosition(16,34, 'W57S44');
    if(creep.memory.prospecting && (!creep.pos.isNearTo(prospect) || creep.carry.energy === creep.carryCapacity)) {
        creep.memory.prospecting = false;
        creep.say('⛏ seeking');
    }
    if(!creep.memory.prospecting && (creep.carry.energy < creep.carryCapacity && creep.pos.isNearTo(prospect))) {
        creep.memory.prospecting = true;
        creep.say('Container');
    }
        if(creep.memory.prospecting) {
   var sources = creep.room.find(FIND_SOURCES);
        if(creep.harvest(sources[0]) == ERR_NOT_IN_RANGE) {
            creep.moveTo(sources[0], {visualizePathStyle: {stroke: '#ffaa00'}});
        }
    }
     if(!creep.memory.prospecting && creep.carry.energy < creep.carryCapacity) {
        creep.moveTo(prospect)
    }
        else if (!creep.memory.prospecting && creep.carry.energy === creep.carryCapacity) {
            creep.moveTo(home);
        var targets = creep.pos.findClosestByPath(FIND_STRUCTURES, {
                filter: (s) => (s.structureType == STRUCTURE_CONTAINER || s.structureType == STRUCTURE_STORAGE) &&
                        s.store.energy < s.storeCapacity
        });
        if(targets) {
            if(creep.transfer(targets, RESOURCE_ENERGY) == ERR_NOT_IN_RANGE) {
                creep.moveTo(targets, {visualizePathStyle: {stroke: '#ffffff'}});
        }
    }
}

@wtfrank @Saruss @famine Thanks a lot for the explanation guys now I understand why it is different and the code is working all good also.

Why is the coding excessively cryptic? It seems that the command which works fine when transferring energy to STRUCTURE_EXTENSION does not also work for STRUCTURE_STORAGE or CONTAINER. Any help would be great, see my current code below:

run: function(creep) {
    if(creep.carry.energy < creep.carryCapacity) {
        var sources = creep.room.find(FIND_SOURCES);
        if(creep.harvest(sources[1]) == ERR_NOT_IN_RANGE) {
            creep.moveTo(sources[1], {visualizePathStyle: {stroke: '#ffaa00'}});
        }
    }
    else {
        var targets = creep.pos.findClosestByPath(FIND_STRUCTURES, {
                filter: (s) => (s.structureType == STRUCTURE_STORAGE) &&
                        s.energy < s.energyCapacity
        });
        if(targets) {
            if(creep.transfer(targets, RESOURCE_ENERGY) == ERR_NOT_IN_RANGE) {
                creep.moveTo(targets, {visualizePathStyle: {stroke: '#ffffff'}});

Thanks a great deal for your help guys, I managed to use your advice to get the harvesters to use both sources in my room currently just be writing sources[1] but I will work on upgrading to a dedicated stationary miner soon. Currently, I am having trouble getting my distance harvester to reliably go to a neighbouring room and harvest a source there. It goes there sometimes (with some variations of the code below but not consistently). If anyone could give me any advice on how to tweak it I would be very grateful:

var roleDistanceharvester = {

/** @param {Creep} creep **/
run: function(creep) {
    if(creep.carry.energy < creep.carryCapacity) {
        var prospect = creep.moveTo(new RoomPosition(20,1, 'W57S45'))};
        
        
        if((creep.pos.isEqualTo(20,1, 'W57S45') || creep.pos.isNearTo(20,1, 'W57S45')) && 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], {visualizePathStyle: {stroke: '#ffaa00'}});
        }
    }

    else {
        if(creep.carry.energy = creep.carryCapacity) {
        const stores = creep.pos.findClosestByPath(FIND_STRUCTURES, {
                filter: (s) => (s.structureType == STRUCTURE_EXTENSION || s.structureType == STRUCTURE_SPAWN || s.structureType == STRUCTURE_CONTAINER || s.structureType == STRUCTURE_STORAGE) &&
                        s.energy < s.energyCapacity});
                        
        var targets = creep.moveTo(new RoomPosition(12,16, 'W57S44'));
        if(stores) {
            if(creep.transfer(stores, RESOURCE_ENERGY) == ERR_NOT_IN_RANGE) {
                creep.moveTo(stores, {visualizePathStyle: {stroke: '#ffffff'}});
            }
        }
    }
 }

} }; module.exports = roleDistanceharvester;

I cannot understand how to use a source's Id to create a designated team of harvesters to go to that resource instead of the one closest to my spawn. Currently, all my harvesters only go to the primary source using this code:

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], {visualizePathStyle: {stroke: '#ffaa00'}});
        }
    }

Please, could someone help me understand how I can store a particular source's Id into memory and then instruct a separate team of harvesters to go to that source? I have created a new role - role.southharvester - but cannot formulate the code to get it to harvest the other source.

@Lucio

I am struggling with this exact same problem. Currently, my creeps only harvest from one resource using creep.room.find(FIND_SOURCES). I want to create a team that harvests the other source in my room but can't find any instruction on how to order the creeps to go to a particular source ID. Did you get any further in sorting this out?