Do loops check the array.length every time when comparing i against array.length?
I was browsing around and I found this:
var i, len;
for(i = 0, len = array.length; i < len; i++) {
//...
}
My first thoughts are:
Do normal loops (the ones that don't cache the length) check the array.length each time?
A loop consisting of three parts is executed as follows:
for (A; B; C)
A - Executed before the enumeration
B - condition to test
C - expression after each enumeration (so, not if B evaluated to false)
So, yes: The .length property of an array is checked at each enumeration if it's constructed as for(var i=0; i<array.length; i++) . For micro-optimisation, it's efficient to store the length of an array in a temporary variable (see also: What's the fastest way to loop through an array in JavaScript?).
Equivalent to for (var i=0; i<array.length; i++) { ... } :
var i = 0;
while (i < array.length) {
...
i++;
}
Is it worth it? (obviously yes, why else he will do it this way?)
Absolutely yes. Because, as you say, loop will calculate array length each time. So this will cause an enormous overhead. Run the following code snippets in your firebug or chrome dev tool vs.
// create an array with 50.000 items
(function(){
window.items = [];
for (var i = 0; i < 50000; i++) {
items.push(i);
}
})();
// a profiler function that will return given function's execution time in milliseconds
var getExecutionTime = function(fn) {
var start = new Date().getTime();
fn();
var end = new Date().getTime();
console.log(end - start);
}
var optimized = function() {
var newItems = [];
for (var i = 0, len = items.length; i < len; i++) {
newItems.push(items[i]);
}
};
var unOptimized = function() {
var newItems= [];
for (var i = 0; i < items.length; i++) {
newItems.push(items[i]);
}
};
getExecutionTime(optimized);
getExecutionTime(unOptimized);
Here is the approximate results in various browsers
Browser optimized unOptimized Firefox 14 26 Chrome 15 32 IE9 22 40 IE8 82 157 IE7 76 148
So consider it again, and use optimized way :)
Note: I tried to work this code on jsPerf but I couldn't access jsPerf now. I guess, it is down when I tried.
if array.length is a calculated value (unlikely in your example) then it is better to retrieve the value one time and store it in a variable.
if array.length is not calculated (likely the case in your example) then it is of no value to retrieve the length and store it in a variable.
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