FOR loop performance in Javascript [duplicate]

Answer 1

From High Performance JavaScript

Decreasing the work per iteration:

//original loops
for (var i=0; i < items.length; i++){
process(items[i]);
}

var j=0;
while (j < items.length){
process(items[j++]]);
}

var k=0;
do {
process(items[k++]);
} while (k < items.length);



//minimizing property lookups
for (var i=0, len=items.length; i < len; i++){
process(items[i]);
}

var j=0,
count = items.length;
while (j < count){
process(items[j++]]);
}

var k=0,
num = items.length;
do {
process(items[k++]);
} while (k < num);


//minimizing property lookups and reversing
for (var i=items.length; i--; ){
process(items[i]);
}

var j = items.length;
while (j--){
process(items[j]]);
}

var k = items.length-1;
do {
process(items[k]);
} while (k--);

Decreasing the number of iterations:

//credit: Jeff Greenberg
var i = items.length % 8;
while(i){
process(items[i--]);
}
i = Math.floor(items.length / 8);
while(i){
process(items[i--]);
process(items[i--]);
process(items[i--]);
process(items[i--]);
process(items[i--]);
process(items[i--]);
process(items[i--]);
process(items[i--]);
}

See JavaScript Optimization