Question

Does make sense to check if the length of the array is not equal to 0 before of to do the loop?

var arr = [1,2,3];
if (arr.length !== 0) {
    // do loop
}

Answer 1

In the general case it doesn’t make sense. For a foreach, for, or while loop, the loop condition will prevent the loop executing anyway, so your check is redundant.

var arr = [];
for (var loopCounter = 0; loopCounter < arr.length; loopCounter++)
{
   // This loop will never be entered.
} 

foreach (var element in arr)
{
   // This loop will never be entered.
}

var loopCounter = 0;
while (loopCounter < arr.length)
{
   // This loop will never be entered.
   loopCounter++;
} 

However, the only time it is important is if you are using a do…while loop. In this scenario, the loop executes, and then you check your loop-condition (when it is too late). It’s likely that your code would have thrown an exception inside the loop in this case, as demonstrated in the following code.

var arr = [];
var loopCounter = 0;
do 
{ 
   someMethod(arr[loopCounter]); // This will throw an error
   loopCounter++;
} 
while(loopCounter < arr.length);