What is this "document.all" line doing?

Question

I'm a Flex/Actionscript refugee and trying to make my way with JS/HTML5/CSS3. Some things make immediate sense but then there are others which just don't.

I am looking at this JSfiddle "Pure Javascript Draggable" and I am not understanding this line (or rather, I understand what it is doing but not how it is doing it)

    x_pos = document.all ? window.event.clientX : e.pageX;

I looked up "document.all" and it seems to be shorthand for Element.querySelectorAll() without arguments?

Returns a non-live NodeList of all elements descended from the element on which it is invoked that match the specified group of CSS selectors.

Is that correct? So the "all" param means it is returning everything in the DOM?
What does a "non-live" NodeList mean? "non-live"?
And the actual line is... testing if window.event.clientX or e.pageX is non-null?

Basic stuff but confusing.

var selected = null, // Object of the element to be moved
    x_pos = 0, y_pos = 0, // Stores x & y coordinates of the mouse pointer
    x_elem = 0, y_elem = 0; // Stores top, left values (edge) of the element

// Will be called when user starts dragging an element
function _drag_init(elem) {
    // Store the object of the element which needs to be moved
    selected = elem;
    x_elem = x_pos - selected.offsetLeft;
    y_elem = y_pos - selected.offsetTop;
}

// Will be called when user dragging an element
function _move_elem(e) {
    x_pos = document.all ? window.event.clientX : e.pageX;
    y_pos = document.all ? window.event.clientY : e.pageY;
    if (selected !== null) {
        selected.style.left = (x_pos - x_elem) + 'px';
        selected.style.top = (y_pos - y_elem) + 'px';
    }
}

// Destroy the object when we are done
function _destroy() {
    selected = null;
}

// Bind the functions...
document.getElementById('draggable-element').onmousedown = function () {
    _drag_init(this);
    return false;
};

document.onmousemove = _move_elem;
document.onmouseup = _destroy;

No, `document.all` is an old Microsoft IE thing. It's basically a (bad) way to detect that the code is running in Internet Explorer. — Pointy, May 01 '16 at 14:01

score 2 · Accepted Answer · edited May 23 '17 at 11:52

Is that correct? So the "all" param means it is returning everything in the DOM?

Yes. Every single one of the elements in the page is put inside a 1-D, array-like structure and handed to you.

What does a "non-live" NodeList mean? "non-live"?

NodeLists can be live or non-live. A live NodeList means the array elements change when the relevant DOM changes (i.e.: If I removed an <a> from the DOM, a live NodeList gathering all <a>s would magically lose one element). A non-live is well... one that doesn't change. (i.e.: If I removed an <a> from the DOM, a non-live NodeList gathering all <a>s would still have reference to the removed <a>).

And the actual line is... testing if window.event.clientX or e.pageX is non-null?

document.all is probably being used to check what browser is being used. In this case, it checks which event object to use, whether the global event or the local event.

maioman · Answer 2 · 2016-05-01T14:15:00.433

document.all?

is checking if the method exists (inside ternary assignment)

What does a "non-live" NodeList mean? "non-live"?

means that if you do actions on the items of the retrieved nodeList that would effect the nodeList you're safe that the list won't change;
(the typical example is selecting by classname and then changing the classname);
if you had obtained your list from getElementsByClassName the list would update right away effecting the list.

that said in your example script you're never using querySelectorAll(),

quoting pointy's comment

'document.all is an old Microsoft IE thing. It's basically a (bad) way to detect that the code is running in Internet Explorer.'

What is this "document.all" line doing?

2 Answers2