performance - Javascript improved native for loop

Question

I have just installed Aptana Studio for development and one of the available commands for Javascript is Insert a for loop like this:

for (var i=0; i < Things.length; i++) {
  Things[i]
};

Another option is Insert improved for loop like this:

for (var i = Things.length - 1; i >= 0; i--){
  Things[i]
};

Why this last one is better than the first one?

Answer 1

//   (  A  )  (       B       )  (C)
for (var i=0; i < Things.length; i++) {
  Things[i]
};

• A is executed once before the loop starts.
• B is re-evaluated before every iteration, and if it's not true, it exits the loop (hence it checks the length property of Things on every single iteration.)
• C is executed after every iteration

That said, the performance you get from changing the loop is minimal, and you risk sacrificing part of the readability, so stick with what you find most readable - not what is most correct performance-wise.

---

This might make more sense for you:

for (var i=0; i < Things.length; i++) {
    Things[i] = null;
};

could be rewritten as the following:

var i = 0; //A
while (true) {
    if (!(i < Things.length)) { //B - We check the length all the time
        break;
    }
    Things[i] = null;
    i++; //C
}

and

for (var i = Things.length - 1; i >= 0; i--){
    Things[i] = null;
};

could be rewritten as the following:

var i = Things.length - 1; //A - We only check the length once
while (true) {
    if (!(i >= 0)) { //B
        break;
    }
    Things[i] = null;
    i--; //C
}