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up vote 0 down vote favorite

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?

share|improve this question
    
an article in case you want to know what "truthiness" is. – OnesimusUnbound Feb 24 '12 at 22:30
    
@OnesimusUnbound yeah that article helps ALOT, thanks! – Jacob Feb 24 '12 at 22:35

4 Answers 4

up vote 7 down vote accepted

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]);
}
share|improve this answer
    
Those examples are all of them :) – pimvdb Feb 24 '12 at 22:36
    
great explanation, thanks alot – Jacob Feb 24 '12 at 22:39
up vote 4 down vote

When i-- reaches 0, the loop stops.

The loop stops once the expression in while ( ... ) evaluates to false.
Since i is a positive number (length), and decreasing, it will reach zero at some point.

In a boolean context, zero translates to false, ensuring that the loop stops at some point.
Because i-- returns the value prior decrement, i is actually -1 when the loop stops.

share|improve this answer
    
It is not quite correct to say that 'zero equals false', better to say that 0 evaluates to false. – vascowhite Feb 24 '12 at 22:25
1  
Zero does not exist in a boolean context, strictly speaking. What's happening is that the abstract ToBoolean function converts 0 to false. – pimvdb Feb 24 '12 at 22:34
    
What he said :) – vascowhite Feb 24 '12 at 22:36
    
In a JS boolean context ("world"), we have only two values: true and false (that's my interpretation of the word "context"). Did I miss anything here? – Rob W Feb 24 '12 at 22:38
    
It looks like we're talking on different wavelengths. If only true and false exist in boolean context and nothing else (as you say), it does not make sense to introduce zero in that context, does it? – pimvdb Feb 24 '12 at 22:39
up vote 1 down vote

Every time the loop runs, i-- is checked. The -- tells Javascript to decrement i by 1. Eventually, i will get down to 0, which is equivalent to false. At this point, the loop stops.

share|improve this answer
up vote 0 down vote

Taking a stab in the dark it's because 0 evaluates to false in a boolean expression. 0 == true returns false 0 == false returns true

Notice how if you start at -1 it will loop endlessly.

share|improve this answer

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