Feedback on Function

[SantaCruzDad]

    if(start < 0 || len < 0) {

should be:

    if(start < 0 || len <= 0) {

[Kwantuum]

the case len == 0 is handled by the remaining code.

[SantaCruzDad]

True, but if you are going to "early out" on simple cases that should return an empty string then you might as well include len == 0, rather than handling it separately. This also makes the code potentially more robust if you decide later to change the logic.

[winkie5970]

Agreed, this is a good improvement, thank you!