I am interested in learning more about the performance attributes of untyped JavaScript arrays, given that they are not homogenous. I wonder how this is handled internally.
For instance, if I were to have a number and an arbitrary object in an array, would they be stored sequentially in memory? Are all primitive data types boxed, with the array simply containing pointers to each element? Is this behavior determined by the VM's implementation?
The behavior varies depending on how the JavaScript engine is implemented.
Typically, in JavaScript arrays, integers and floats are stored by their values while all other objects are stored by reference.
For example, in V8, arrays can be either of type PACKED_ELEMENTS or HOLEY_ELEMENTS, with each string being stored separately on the heap.
To confirm this, you can utilize the %DebugPrint function in a debug version of the V8 engine (obtainable using the jsvu tool):
d8> var a = [1, 2, 'aaa']; %DebugPrint(a);
DebugPrint: 000003B13FECFC89: [JSArray]
- elements: 0x03b13fecfc31 <FixedArray[3]> {
0: 1
1: 2
2: 0x00c73b3e0fe1 <String[#3]: aaa>
}
Upon investigation, it appears that Ryan Peden has thoroughly examined all the intriguing specifics (and quite recently):
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