Javascript - No data without timeout


I have a problem with javascript and I would like to understand why and how it works.

I have an object that uses an API. In this object I have an attribute of a list type of an object.

My objects are well informed with the data.
But when after I try to access my object list, I can’t.
If I do
console.log (myObjet.lstOtherObject);
My object list is displayed.
If I do
console.log (myObjet.lstOtherObject[0]);
It shows me undefined.

If I put my code in a setTimeout I have no problem.

When I set my list

 addLight(light: Light) : boolean{
        if(this._lstLights.find(e => e.uniqueid === light.uniqueid)){
            return false;
            return true;

What can be the problem ?
Thanx for your response