Where do you see 4 bits? I only see him mention 3 bits: The old jsval representation fit in a 32 bit value, using the 3 lowest bits as a way to tag the value a

Where do you see 4 bits? I only see him mention 3 bits: The old jsval representation fit in a 32 bit value, using the 3 lowest bits as a way to tag the value as a particular type. These were called type tags. In the examples given, the lowest three bits are masked to 0s when determining the address. That's a loss of precision, meaning there's no way to represent a pointer to an address that's not modulo 8 == 0. Another way of saying this is that objects must be aligned to 8 byte boundaries. Isn't this a waste of space? Probably not. My guess is very few objects are less than 8 bytes in length, so that space is not really wasted. Even if objects ARE shorter, this is a dynamically typed language, we probably want to keep spare memory anyway, to reduce the chance that adding values to an object will require costly memory allocation.