Question

This is Peter Higgins’s pub sub library: https://github.com/phiggins42/bloody-jquery-plugins/blob/master/pubsub.js

(function (d) {
    var cache = {};

    d.publish = function (topic, args) {
        cache[topic] && d.each(cache[topic], function () {
                this.apply(d, args || []);
        });
    };

    d.subscribe = function (topic, callback) {
        if (!cache[topic]) {
            cache[topic] = [];
        }
        cache[topic].push(callback);
        return [topic, callback];
    };

    d.unsubscribe = function (handle) {
        var t = handle[0];
        cache[t] && d.each(cache[t], function (idx) {
            if (this == handle[1]) {
                cache[t].splice(idx, 1);
            }
        });
    };

})(jQuery);

I don’t understand the logic and the functionality of publish:

cache[topic] && d.each(cache[topic], function () {
    **this.apply(d, args || []);** //what is happening here?
});

What is the purpose of this part? except the fact that it publishes the event

Answer 1

In this context, the && is used as a shorthand for:

if (cache[topic]) {
    d.each(cache[topic], function() { … });
}

This is because && (and ||) are short-circuiting, so if the left hand side evaluates to a false-ish value (or true-ish value, in the case of ||), the right hand side does not get evaluated.

For example:

> function foo(result) { console.log("foo"); return result; }
> function bar(result) { console.log("bar"); return result; }
> foo(false) && bar(true);
foo
false