On x64 in Firefox, at least, there are no bounds checks; the index is a uint32; the entire accessible 4GB range is mapped PROT_NONE with only the accessible region mapped PROT_READ|PROT_WRITE; out-of-bounds accesses thus reliably turn into SIGSEGVs which are handled safely after which execution resumes. Thus, bounds checking is effectively performed by the MMU.