Can anybody explain the following JavaScript code from jCarousel?

Question

I've been writing JS for a while, and I have been trying to debug the jQuery jCarousel plugin for IE7 compatibility. A lot of the variables are obfuscated, which is OK, but I have never seen syntax like this:

scroll: function (a, c) {
    !this.locked && !this.animating && (this.pauseAuto(), this.animate(this.pos(a), c))
}

It seems like some sort of shorthand notation, but I've never come across anything like this.
Similarly,

for (var a = function (a) {
    i.get(a).each(function () {
        h(i, this, a, b, c)
    })
}, k = d; k <= f; k++)  {
    k !== null && !(k >= j && k <= e) && a(k)
}

I have never seen a function being assigned as the iterator, and again the block statement inside looks like the first example. I know we all strive to save a few bytes when we write our code, but to me this feverish attempt comes at a cost of utter confusion and bewilderment to other programmers. Can anybody can give me a simple, "longhand" alternative / thorough explanation of what's happening? I always want to know more about this language and how it works, thanks.

Answer 1

The && operator evaluates each expression until it finds one which is truthy, which it returns. If none are truthy, it returns the last one.

The , operator evaluates both it's operands and returns the second one (no matter what they return).

So in the first example, it's basically saying:

scroll: function (a, c) {
    if (!this.locked && !this.animating) {
        this.pauseAuto();
        this.animate(this.pos(a), c)
    }
}

As for the second example, it's important to note the iteration is over the k variable, not a :

var a = function (a) {
    i.get(a).each(function () {
        h(i, this, a, b, c)
    })
};

for (k = d; k <= f; k++)  {
    if (k !== null && !(k >= j && k <= e)) {
        a(k)
    }
}

For the second example, don't forget it's possible to define multiple variables in one var statement using the , :

var a = 1,
    b = 2,
    c = 3;

---

It's important to know that the developer himself isn't minimizing the code like this; he's writing normal source code with meaningful variable names in easy to read blocks. When it comes to release time, he'll run the minified version through a minifier ( UglifyJS , Closure Compiler etc) to get the reduced code.

Answer 2

Because of short-circuit evaluation:

a && b && (c, d, e);

is basically equivalent to:

if (a) {
    if (b) {
        c;
        d;
        e;
    }
}

And the value of the expression will be e .

In the second example, the function is not an iterator, he is just using the first position of the for loop to initialize an anonymous function and then assign it to a .

The author of that code is just being overly terse. Just because you can, doesn't mean you should.

Update :

Here is a re-write of the second example that should be functionally equivalent:

function a (a) {
    i.get(a).each(function () {
        h(i, this, a, b, c)
    });
}

for (k = d; k <= f; k++)  {
    if ((k !== null) && (!(k >= j && k <= e))) {
        a(k);
    }
}