In JS how do you know the right condition to use inside your for loop?

I'm new to JavaScript and I've been struggling with iterating with for loops.

The early classes were easy to understand when the condition was simply i < 9 or i > 9.

The issue I have now is knowing when to use conditions like i < arr.length for example.

Many times when I do JS challenges and check the solution for challenges I fail, I see these type of conditions and I don't know why one condition is used instead of another.

Can anyone help me out with this, please?