“ document.all”这一行在做什么？

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;

Answer 1

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 (ie: 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. (ie: 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.

Answer 2

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.'