Question

I am using Addy Osmani’s pub sub method:

var topics = {};

jQuery.Topic = function( id ) {
    var callbacks,
        topic = id && topics[ id ];
    if ( !topic ) {
        callbacks = jQuery.Callbacks();
        topic = {
            publish: callbacks.fire,
            subscribe: callbacks.add,
            unsubscribe: callbacks.remove
        };
        if ( id ) {
            topics[ id ] = topic;
        }
    }
    return topic;
};

but before I add it to my project I’d like to understand it a little better. It’s fairly simple except for one line I’ve never seen: topic = id && topics[ id ];

topic = id ok. What is the && operator doing here? is it adding id to the topics array? or making a comparison?

Answer 1

topic = id && topics[ id ]; 

should be read as

topic = (id && topics[ id ]);

&& has an higher precedence, as stated on MDN documentation

The value assigned to topic is topic[id] when id is evaluated as “true” value, false otherwise: so this is a shorter way for

if (id) {
   topic = topics[id];
}
else {
   topic = false;
}