As I understand it, V8 keeps track of an ElementsKind for each array (or, more precisely, for the elements of every object; arrays are not special in this sense

As I understand it, V8 keeps track of an ElementsKind for each array (or, more precisely, for the elements of every object; arrays are not special in this sense). If an array only contains floats, then they will all be stored unboxed and inline. See here: https://source.chromium.org/chromium/chromium/src/+/main:v8/src/objects/elements-kind.h;l=78;bpv=1 https://source.chromium.org/chromium/chromium/src/+/main:v8/... I assume that integers are coerced to floats in this mode, and that there's a performance cliff if you store a non-number in such an array, but in both cases I'm just guessing. In SpiderMonkey, as you say, we store all our values as doubles, and disguise the non-float values as NaNs.