Thank you both for your answers. That really cleared things up!

As a test, I created the following code:

if (!MyClass) {
    console.log('define class');
    var MyClass = class {
        run() {
            console.log('AHAH');
        }
    }
}

module.exports.loop = function () {
    console.log('tick');
    let spawn = Game.spawns['Spawn1'];
    
    if(!spawn.spawning) {
        console.log('spawn');
        spawn.spawnCreep([MOVE], 'myCreep', { memory: { myObj: new MyClass() } });
    }
    
    for (let creep in Game.creeps) {
        console.log(creep, JSON.stringify(Game.creeps[creep].memory));
        Game.creeps[creep].memory.myObj.run();
    }
}

This is the output:

[00:44:03]define class
[00:44:03]tick
[00:44:03]spawn
[00:44:03]myCreep {"myObj":{}}
[00:44:03]AHAH
[00:44:04]tick
[00:44:04]myCreep {"myObj":{}}
[00:44:04]TypeError: Game.creeps[creep].memory.myObj.run is not a function
    at Object.module.exports.loop:25:41
    at __mainLoop:1:19079
    at eval:2:4
    at Object.r.run:2:143185
[00:44:05]tick
[00:44:05]myCreep {"myObj":{}}
[00:44:06]TypeError: Game.creeps[creep].memory.myObj.run is not a function
    at Object.module.exports.loop:25:41
    at __mainLoop:1:19079
    at eval:2:4
    at Object.r.run:2:143185
[00:44:15]define class
[00:44:15]tick
[00:44:15]spawn
[00:44:15]myCreep {"myObj":{}}
[00:44:15]TypeError: Game.creeps[creep].memory.myObj.run is not a function
    at Object.module.exports.loop:25:41
    at __mainLoop:1:19079
    at eval:2:4
    at Object.r.run:2:143185
[00:44:30]define class
[00:44:30]tick
[00:44:30]spawn
[00:44:30]myCreep {"myObj":{}}
[00:44:30]TypeError: Game.creeps[creep].memory.myObj.run is not a function
    at Object.module.exports.loop:25:41
    at __mainLoop:1:19079
    at eval:2:4
    at Object.r.run:2:143185
[00:44:32]define class
[00:44:32]tick
[00:44:32]spawn
[00:44:32]myCreep {"myObj":{}}
[00:44:32]TypeError: Game.creeps[creep].memory.myObj.run is not a function
    at Object.module.exports.loop:25:41
    at __mainLoop:1:19079
    at eval:2:4
    at Object.r.run:2:143185
[00:44:33]tick
[00:44:33]spawn
[00:44:33]myCreep {"myObj":{}}
[00:44:33]TypeError: Game.creeps[creep].memory.myObj.run is not a function
    at Object.module.exports.loop:25:41
    at __mainLoop:1:19079
    at eval:2:4
    at Object.r.run:2:143185
[00:44:42]define class
[00:44:42]tick
[00:44:42]spawn
[00:44:42]myCreep {"myObj":{}}
[00:44:42]TypeError: Game.creeps[creep].memory.myObj.run is not a function
    at Object.module.exports.loop:25:41
    at __mainLoop:1:19079
    at eval:2:4
    at Object.r.run:2:143185
[00:44:50]tick
[00:44:50]spawn
[00:44:50]myCreep {"myObj":{}}
[00:44:50]TypeError: Game.creeps[creep].memory.myObj.run is not a function
    at Object.module.exports.loop:25:41
    at __mainLoop:1:19079
    at eval:2:4
    at Object.r.run:2:143185
[00:44:51]tick
[00:44:51]spawn
[00:44:51]myCreep {"myObj":{}}
[00:44:51]TypeError: Game.creeps[creep].memory.myObj.run is not a function
    at Object.module.exports.loop:25:41
    at __mainLoop:1:19079
    at eval:2:4
    at Object.r.run:2:143185

I have several questions about the behaviour of this code.

The first is that it seems that the object myObj forgets all his members the next tick after it started spawning. Why is that? Does this has something to do with the lifetime of the MyClass class object or something (I'm a JS newbie)? How do I deal with this? Storing classes themselves in memory?

Secondly, and this question propably ties in with the first, why is MyClass defined just sometimes? Why not either just once or always?