Need help in converting C++ to javascript

Question

I'm constructing a javascript indicator for my client and they gave me below C++ code from their old system. I have never done C++ program before. Below is the part of the C++ code. What I want to know is in the line
if (it3 != d1Swing.end() && it3->x == h[i].x) --(it1 = it2 = it3); what is the meaning of --(it1 = it2 = it3)? What will it looks like in javascript?

vector<PTPoint::PTIndexPoint> dnSwing;
list<PTPoint::PTIndexPoint> hq, lq;
vector<PTPoint::PTIndexPoint>::iterator it1 = d1Swing.begin(), it2 = d1Swing.begin(), it3 = ++d1Swing.begin();

//
// more code here
//
for (int i = 0; i < period; ++i)
{
    while (!hq.empty() && hq.back().y < h[i].y) hq.pop_back();
    hq.push_back(h[i]);
    while (!lq.empty() && lq.back().y > l[i].y) lq.pop_back();
    lq.push_back(l[i]);

    if (it3 != d1Swing.end() && it3->x == h[i].x) --(it1 = it2 = it3);
    //
    // more code here
    //
} 

//
// more code here
//
p->swap(dnSwing);

Thanks in advance.

tslin

Answer 1

It means that their previous programmer loved being "clever".

The value of an assignment is a reference to the object that was assigned to, and assignment associates to the right.

--(it1 = it2 = it3)

is

--(it1 = (it2 = it3))

and it's intended to assign the value of it3 to it2 and it1 , then decrement it1 .
(I have a hunch that this may be undefined, which is a thing that happens frequently when you're being clever in C++.)

it1 is apparently intended to be "one step behind" it2 .

A more reasonable way to write that is

it2 = it3;
it1 = it2 - 1;

(In JavaScript, I suspect that you need to work with array indices rather than iterators to accomplish the same thing.)