tschneidereit
Memory order?
The given example var PointType = new StructType({x: float64, y: float64}) uses an object to specify which types are supposed to be used and what their attribute keys are. But object keys are unordered in JS, so it's unclear in which order they are "[...] laid out in memory consecutively"[1]. Or am I overlooking something here?
The order will usually not matter, but it may in serialization.
Was just checking with @dherman. He clarified that the intention of ES6 was to fully specify the order in those cases, but that it was something we ought to revisit to see what precisely wound up in the spec and make sure that it is guaranteed, or at least guaranteed enough.
Maps do preserve insertion order: https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/Global_Objects/Map
It's not quite clear for objects. The ECMA-262 v3.0 specs explicitly stated, that the order is not guaranteed (4.3.3). In v5.1 this sentence is gone, but there's still a note on the unordered nature found under for-in (12.6.4). They removed it in v6.0, but under Object.keys() you can find (19.1.2.14):
If an implementation defines a specific order of enumeration for the for-in statement, the same order must be used for the elements of the array returned in step 4.
The "if" is quite irritating.
This is why the property names / types should probably be specified via an array if users want explicit ordering, e.g.
var PointType = new StructType([['x', float64], ['y', float64]]);
If the user does not specify the order, it should be up to the engine to order them in the most space efficient way - e.g. for alignment reasons you probably don't want an object to have a layout like:
int16, int32, int16
when you can have:
int16, int16, int32
