I've noticed when using setInterval() with objects, it accepts this:

setInterval(function(){auto1.increaseValue(s);}, auto1.interval);

Where auto1 is an object from a class. However, the program throws an error when I put an object or object of an array of another object inside the function. Ex.

    let index = this.arrayForItems.length-1;
    setInterval(function(){this.arrayForItems[index].increaseValue(s);header.innerHTML = s.value;}, this.arrayForItems[index].interval);

    setInterval(function(){this.item.increaseValue(s);header.innerHTML = s.value;}, this.item.interval);//Where item is the object inside object shop

But this works.

    let index = this.arrayForItems.length-1;
    let referenceToPurchase = this.arrayForItems[index];
    setInterval(function(){referenceToPurchase.increaseValue(s);header.innerHTML = s.value;}, referenceToPurchase.interval);

Why is this so?