Is the condition in the for loop evaluated in every loop?

Question

I have a situation like:

for(var i = 0; i < a + b; ++i)
  // code that doesn't affect a and b

Should I worry about the addition been performed in every iteration? Or JavaScript (its parser?) is smart enough to understand that a + b is constant?

In other words, should I do that like this:

var end = a + b;
for(var i = 0; i < end; ++i)
  // code

or will this waste a line of code?

---

Well, actually what I worry about is not that one line of code, BUT the fact that I am thinking about it every time I face a situation like this in JavaScript! Also, today it's an addition, tomorrow it may be something else, like the square root of it, so I think it's important!

Answer 1

It's better to define the constant like this :

for(var i = 0, end = a + b; i < end; ++i)

---

Optimization way

You can write your loop like this :

for(var i = a + b; i--;)

This is for me the more optimized but i is descending and not ascending

---

More example

A simple example to understand. If you create your loop like this :

for(var i = 0; i < array.length; ++i) { 
   array.push('value'); // infinite loop
}

array.length is evaluated on every iteration and you can create an infinite loop with array.push().