JavaScript For Loop Efficiency – Best Practices

javascript

for (var i=0, cols=columns.length; i<cols; i++) { ... }

more efficient than

for (var i=0; i<columns.length; i++) { ... }

In the second variant, is columns.length calculated each time the condition i<columns.length is checked ?

Best Answer

Any expression that's in the second portion of a for will be evaluated once per loop.

So, here, with your second proposition, yes, columns.length will be calculated each time the condition is checked -- which would make the first proposition faster than the second one.

(That's true for many other languages, btw)

Related Solutions

JavaScript – Improved Native For Loop Performance

//   (  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
}

Best Answer

Related Solutions

JavaScript – Improved Native For Loop Performance

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