VMs with a direct asm.js compilation mode (like SpiderMonkey) can compile directly to native code (in advance) with the vast majority of the JS attack surface disabled (including heap allocations, for asm.js v1)
In practice it's similar to the way NACL/pNACL can sandbox code due to the limited instruction set & validation guarantees.
Comments
VMs with a direct asm.js compilation mode (like SpiderMonkey) can compile directly to native code (in advance) with the vast majority of the JS attack surface disabled (including heap allocations, for asm.js v1)
In practice it's similar to the way NACL/pNACL can sandbox code due to the limited instruction set & validation guarantees.