Let's compare two variations of a loop iteration:
for (var i = 0; i < nodes.length; i++) {
...
}
and
var len = nodes.length;
for (var i = 0; i < len; i++) {
...
}
Would the second version be faster than the first one in any way?
Let's compare two variations of a loop iteration:
for (var i = 0; i < nodes.length; i++) {
...
}
and
var len = nodes.length;
for (var i = 0; i < len; i++) {
...
}
Would the second version be faster than the first one in any way?
It is inaccurate to claim that the proposed solution is correct, as any decent engine should have the ability to optimize the loop by moving the property load outside of it.
A test on this jsperf reveals an interesting observation in V8 regarding register allocation when storing variables versus using them directly inside a loop. This phenomenon likely occurs in SpiderMonkey and Opera as well.
The presenter asserts in their presentation that JSPerf evaluations are frequently incorrect, leading to misleading conclusions drawn from faulty tests like those referenced in this thread.
Warning signs include placing code within test cases rather than functions, neglecting result verification or dead code elimination, defining functions locally instead of globally, and failing to warm up test functions before benchmarks to prevent compilation during timed sections for consistency.
Update: 16/12/2015
Since this answer continues to receive a significant amount of views, I have decided to revisit the issue considering the advancements in browsers and JavaScript engines.
Instead of relying on JSPerf, I have created some sample code to demonstrate looping through arrays using the methods mentioned in the original question. The code has been organized into functions to simulate real-world application scenarios:
function getTestArray(numEntries) {
var testArray = [];
for (var i = 0; i < numEntries; i++) {
testArray.push(Math.random());
}
return testArray;
}
function testInVariable(testArray) {
for (var i = 0; i < testArray.length; i++) {
doSomethingAwesome(testArray[i]);
}
}
function testInLoop(testArray) {
var len = testArray.length;
for (var i = 0; i < len; i++) {
doSomethingAwesome(testArray[i]);
}
}
function doSomethingAwesome(i) {
return i + 2;
}
function runAndAverageTest(testToRun, testArray, numTimesToRun) {
var totalTime = 0;
for (var i = 0; i < numTimesToRun; i++) {
var start = new Date();
testToRun(testArray);
var end = new Date();
totalTime += (end - start);
}
return totalTime / numTimesToRun;
}
function runTests() {
var smallTestArray = getTestArray(10000);
var largeTestArray = getTestArray(10000000);
var smallTestInLoop = runAndAverageTest(testInLoop, smallTestArray, 5);
var largeTestInLoop = runAndAverageTest(testInLoop, largeTestArray, 5);
var smallTestVariable = runAndAverageTest(testInVariable, smallTestArray, 5);
var largeTestVariable = runAndAverageTest(testInVariable, largeTestArray, 5);
console.log("Length in for statement (small array): " + smallTestInLoop + "ms");
console.log("Length in for statement (large array): " + largeTestInLoop + "ms");
console.log("Length in variable (small array): " + smallTestVariable + "ms");
console.log("Length in variable (large array): " + largeTestVariable + "ms");
}
console.log("Iteration 1");
runTests();
console.log("Iteration 2");
runTests();
console.log("Iteration 3");
runTests();To ensure a fair comparison, each test is conducted 5 times and the results are averaged. Additionally, I have run the entire test, including array generation, three times. Testing on Chrome indicated that both methods resulted in almost identical execution times.
It's important to note that this example serves as a simplified demonstration and may not reflect real-world performance due to other factors influencing the application. For accurate results, it's recommended to conduct tests within the context of your specific application.
The key takeaway
For optimal performance tailored to your application, it is best to conduct your own tests. Given the constant evolution of JS engines, browser technology, and CPU capabilities, testing within your application's context is crucial. Consider whether micro-optimizations truly benefit your users or if time could be better spent improving functionality and addressing bugs for a more positive user experience :).
Original Answer:
The method storing length in a variable would typically result in slightly faster performance. The length property avoids iterating over the array to count elements but requires dereferencing the array each time it is accessed. By storing the length in a variable, the array dereference is eliminated during each iteration of the loop.
If interested in comparing different approaches to looping through JavaScript arrays, refer to this jsperf benchmark.
As per the guidance from w3schools "Reduce Activity in Loops", the below code is deemed as inefficient:
for (i = 0; i < arr.length; i++) {
On the other hand, the following code is considered optimized:
var arrLength = arr.length;
for (i = 0; i < arrLength; i++) {
To test the hypothesis that accessing the DOM is slow, the following experiment was devised:
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8>
<title>my test scripts<</title>
</head>
<body>
<button onclick="initArray()">Init Large Array</button>
<button onclick="iterateArraySlowly()">Iterate Large Array Slowly</button>
<button onclick="iterateArrayQuickly()">Iterate Large Array Quickly</button>
<p id="slow">Slow Time: </p>
<p id="fast">Fast Time: </p>
<p id="access"></p>
<script>
var myArray = [];
function initArray(){
var length = 1e6;
var i;
for(i = 0; i < length; i++) {
myArray[i] = i;
}
console.log("array size: " + myArray.length);
}
function iterateArraySlowly() {
var t0 = new Date().getTime();
var slowText = "Slow Time: "
var i, t;
var elm = document.getElementById("slow");
for (i = 0; i < myArray.length; i++) {
document.getElementById("access").innerHTML = "Value: " + i;
}
t = new Date().getTime() - t0;
elm.innerHTML = slowText + t + "ms";
}
function iterateArrayQuickly() {
var t0 = new Date().getTime();
var fastText = "Fast Time: "
var i, t;
var elm = document.getElementById("fast");
var length = myArray.length;
for (i = 0; i < length; i++) {
document.getElementById("access").innerHTML = "Value: " + i;
}
t = new Date().getTime() - t0;
elm.innerHTML = fastText + t + "ms";
}
</script>
</body>
</html>The results show that the iteration executed first tends to perform better initially. However, after multiple executions, the supposedly "bad code" prevails most of the time. The reasoning behind this phenomenon might require a more in-depth analysis. For now, I prefer sticking to code readability by utilizing the following syntax:
for (i = 0; i < arr.length; i++) {
When dealing with a nodes that is a DOM nodeList, the second loop will outperform the first loop significantly. This is due to the fact that in the first loop, you are constantly looking up DOM elements at each iteration which is very expensive. Check out this jsperf test for more information.
Whenever I run benchmark tests, this consistently outperforms any other method.
for (index = 0, value; value = elements[index]; index++) {
performExploits(value);
}
In my opinion, the nodes.length variable is already set and not recomputed every time it's used. Therefore, the initial example would likely be faster as it requires one less variable declaration. However, any speed difference between the two would probably not be significant.
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