Re: Questions about 'new' : is this pattern legit?

Thomas 'PointedEars' Lahn <PointedEars@xxxxxx> writes:

To nitpick out of context:

2. Several function calls will be always more expensive than just one and
the creation of an Array instance.

That's not a given.

It's possible that four function calls take longer than creating an Array
and calling one function that loops over the array and extracts the elements
again (possible, but not given), but the latter also allocates space for the
array. If this happens often enough, the memory overhead will also cause
earlier garbage collection.

After writing that, I decided a test was in order, instead of relying on
intuition. The following code is attempted written without obvious
inefficiencies in either test case (all variables are local, loop
invariant expressions are lifted, etc.).
I have run it in Opera 10.51, Chrome 5(dev), Firefox 3.6.2 and IE 9 preview.

<script type="text/javascript">
function Obj() {
this.props = {};
var self = this;
this.setChained = function setProp(name, value) {
self.props[name] = value;
return setProp;
};
};
Obj.prototype.setProp = function (name,value) {
this.props[name] = value;
};
Obj.prototype.setProps = function (arr) {
var props = this.props;
for (var i = 0, n = arr.length; i < n; i += 2) {
props[arr[i]] = props[arr[i+1]];
}
};

function test1() {
var o = new Obj();
var t0 = new Date();
for (var i = 0; i < 100000; i++) {
o.setProp("a", i);
o.setProp("b", i);
o.setProp("c", i);
o.setProp("d", i);
}
var t1 = new Date();
return t1-t0;
}

function test2() {
var o = new Obj();
var t0 = new Date();
for (var i = 0; i < 100000; i++) {
o.setChained("a", i)("b", i)("c", i)("d", i);
}
var t1 = new Date();
return t1-t0;
}

function test3() {
var o = new Obj();
var t0 = new Date();
for (var i = 0; i < 100000; i++) {
o.setProps(["a", i, "b", i, "c", i, "d", i]);
}
var t1 = new Date();
return t1-t0;
}

alert([test1(), test2(), test3()]);
</script>


In the first three browsers (not IE), doing four calls is consistently
faster than creating one eight-element array and doing one call. And
not by a small amount - it is between 1.75 and 3 times as fast.
In IE9, they take about the same time.

Using the chained way of calling the setter is about the same as just
doing the four calls in Firefox and Chrome, and somewhat slower in
Opera, but not as slow as using an Array. In IE9, this approach is
actually the fastest. Go figure.

In summary: Function calls aren't significantly slower than other
language features like object creation and property lookup.
Object creation, and inspection, spends a lot of time in memory
access, where function calls probably uses the stack (which is
very fast memory, because it's almost always in the first level
cache of the processor).

/L
--
Lasse Reichstein Holst Nielsen
'Javascript frameworks is a disruptive technology'

.

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