Question

In JavaScript, I have recently came across how reverse while loops are faster.

I have seen them in this form:

var i = someArray.length;

while (i--) {
  console.log(someArray[i]);
}

I tested this out and it stopped once it went through the whole array.

How does it know when to stop once it completes the array?

Answer 1

A while loop evaluates the expression inside the parentheses each time through the loop. When that expression gets to a falsey value, the loop will stop.

Examples of falsey values are:

false 
0  
undefined  
NaN
null
""

In this case the value of i will be decremented each time through the loop and when it hits the value of 0, the loop will stop. Because it’s a post decrement operator, the value of the expression is checked before the decrement. This means that the inner loop will see values of i from someArray.length - 1 to 0 (inclusive) which are all the indexes of that array.

Your code example:

var i = someArray.length;

while (i--) {
    console.log(someArray[i]);
}

creates the same output as this:

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