On 10/28/13 8:50 PM, Allen Wirfs-Brock wrote:
So what's so onerous about returning a fresh array from the getter each time it
was called. If it was implemented in Es6, it would just be:
return Array.from(internal_compy);
Implementing this is pretty easy, for sure.
The questions are:
1) What the object-identity expectations of callers are.
2) Whether this can cause nasty O(N^2) behavior when people loop over
the property's value in a naive way. Mostly an issue if the list can be
long, so not a problem for the gamepad API but could be a problem for
other use cases.
Proxies aren't cheap. Unless these arrays tend to be quite large, I wouldn't be
surprised if the overhead of using a Proxy for this useless ended up being
higher than the cost of making fresh arrays.
Maybe. Measuring in Firefox [1] a get on a proxy seems to be in the
~100ns range for me, while duplicating a small array (3 entries) via
slice.call() seems to be about 300ns (mostly the call overhead).
Duplicating an array with ~100 entries takes closer to 2000ns, just to
see the scaling here.
-Boris
[1] Testcase:
<!DOCTYPE html>
<body>
<pre><script>
var count = 4000000;
var arr = [1, 2, 3];
var slice = Array.prototype.slice;
var proxy = new Proxy(arr, {});
var sideEffect;
var i;
var start = new Date;
for (i = 0; i < count; ++i) {
}
var stop = new Date;
document.writeln("Empty loop: " + (stop - start) / count * 1e6 + "ns");
var start = new Date;
for (var i = 0; i < count; ++i) {
sideEffect = arr[0];
}
var stop = new Date;
document.writeln("Normal array get: " + (stop - start) / count *
1e6 + "ns");
var start = new Date;
for (var i = 0; i < count; ++i) {
sideEffect = proxy[0];
}
var stop = new Date;
document.writeln("Proxy get: " + (stop - start) / count * 1e6 + "ns");
var start = new Date;
for (var i = 0; i < count; ++i) {
sideEffect = slice.call(arr);
}
var stop = new Date;
document.writeln("Array creation, length " + arr.length +": " +
(stop - start) / count * 1e6 + "ns");
arr = arr.concat(arr);
arr = arr.concat(arr);
arr = arr.concat(arr);
arr = arr.concat(arr);
arr = arr.concat(arr);
var start = new Date;
for (var i = 0; i < count; ++i) {
sideEffect = slice.call(arr);
}
var stop = new Date;
document.writeln("Array creation, length " + arr.length +": " +
(stop - start) / count * 1e6 + "ns");
</script>
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