The mitigation for this is: [ ] subscript operator except when the subscript is a numeric literal or string literal or an expression that must produce a number

The mitigation for this is: [ ] subscript operator except when the subscript is a numeric literal or string literal or an expression that must produce a number value. So expressions like a[i] are disallowed because we cannot statically determine that i is not one of the dangerous property names. But a[+i] is allowed, because +i will always produce a number.