Question

So i have known for a while that

for ( var i=0, len = myArray.length; i < len; i++ ){

}

is more efficient than

for ( var i=0; myArray.length; i++ ){

}

for large arrays. But have wondered how much of a performance gain does the former really give you ?

Today i set out to do some benchmarks. I created an array and pushed 100,000 numbers into it.

    var array = []; 
    for( var i = 0; i < 100000; i++ ) 
         array.push(i);  

I then tested both loops above by doing a console.log of each of the numbers in the array and timing the process.

console.time('loop'); 

for( var i = 0; i < array.length; i++ )
     console.log(i); 

console.timeEnd('loop');  

And for the second test

console.time('loop'); 

 for( var i = 0, len = array.length; i < len ;i++ )
     console.log(i) 

console.timeEnd('loop') 

After testing this a few times, my results are inconclusive. I get both high and low numbers for both test cases. So my question is, what’s a better test to show unequivocally that getting the length beforehand has performance benefits and what kind of percentage gain is there to doing this ?

Answer 1

Here is a relevant jsperf:
http://jsperf.com/caching-array-length/4
which shows that the difference can be surprisingly little.