pxt-calliope/docs/reference/js/object-disclaimer.md

Objects disclaimer

Touch Develop docs for the micro:bit.

Memory management on the micro-bit

New user-defined object types are allocated on the heap and are ref-counted. Our ref-counting algorithm does not have a cycle collector. (See Wikipedia.)

Practical consequences

This means that the following script will result in an out-of-memory error.

while (true) {
    let l = new List()
    l.next = l
}

How to avoid this?

Don't do it. It's bad.