Question

Line 4140 of jQuery 1.7rc1:

while ( (elem = elem.previousSibling) && elem.nodeType !== 1 ) {}

This is weird to my eyes. Is it equivalent to

while( elem && elem.nodeType !== 1 ) elem = elem.previousSibling;?

Answer 1

No, it is not equivalent to what you have. The original code:

while ( (elem = elem.previousSibling) && elem.nodeType !== 1 ) {}

is equivalent to this:

while (elem.previousSibling && elem.previousSibling.nodeType !== 1) {
    elem = elem.previousSibling;
    // other code here
}

Logically, it says to loop while there is a previousSibling and while the previousSibling's nodeType !== 1 and assign elem to that previousSibling.

The assignment in the original while condition is a shortcut that saves a separate assignment and saves an extra reference level in the nodeType check.

Breaking down the while loop even more, it’s the same as this:

while (elem.previousSibling) {
    elem = elem.previousSibling;
    if (elem.nodeType !== 1) {
        break;
    }
    // other code here
}

This part:

(elem = elem.previousSibling)

assigns elem.previousSibling to elem and then evaluates whether elem is truthy or not.