for循环中的条件语句像我一样简单

Question

I found these lines of code in a library made to listen to touch events :

  for ( var i = elements.length; i; i-- ) {
    iterator( elements[ i - 1 ], elements );
  }

It surprised me for a second. I understand the code this way : i is an integer positive or null and the conditional statement is if i positive.
Do I understand right ? Does i return false when i==-1 ? Or did i miss something (like it's possible to write a for loop without a conditional statement) ?

Answer 1

The conditional is essentially if( i) proceedToNextIteration();

As numbers go, only zero is falsy and would fail the check. -1 would be true in the above condition.

For this reason, I'd rewrite that code as:

for( var i = elements.length-1; i >= 0; i--) {
    iterator( elements[i], elements);
}

It's much more explicit about its final condition.

Answer 2

This is the structure of the for statement in Javascript:

for ([initialization]; [condition]; [final-expression])
   statement

The statement you provided for ( var i = elements.length; i; i-- )

adheres to this syntax. This is same as

for(var i = 10; i!=0; i--)

Meaning loop from 10 ( elements.length ) to 1 . Similar to looping from 1 to 10 .

In Javascript, the condition i != 0 can be replaced with just i .

Meaning:

if(i != 0)

and

if (i)

are the same. That is what is done in the condition part.

Looping over elements of a list or array from max to 0 is helpful when removing items from it.