assigning function to variables and assigning variables to functions(?) in Javascript

Question

I'm learning javascript by dissecting a few scripts written by other people that I have used before (stackoverflow is essential to this!). This bit of code (and I cut a lot from the middles to make the code easier to read, and all comments are mine) comes from fullCalendar.js , a jquery plugin.

I'm not sure what the author is doing here:

function EventManager(options, _sources) { //called using .call()
    var t = this;

    t.isFetchNeeded = isFetchNeeded;  //?? assign the function "isFetchNeeded" to the variable (not the value returned by the function)??

    var trigger = t.trigger;    //?? namespace??
}

function View(element, calendar, viewName) {
    var t = this;   

    function trigger(name, thisObj) {
        return calendar.trigger.apply(
                 calendar,
                 [name, thisObj || t].concat(Array.prototype.slice.call(arguments, 2), [t])
        );
    }
}

First, my guess is that this.foo=bar is assigning the function "bar" to the variable called "isFetchNeeded" within the current object (not yet sure why you'd do this. speed?).

Second, the line var foo=this.bar has me stumped. It looks like it might be some namespace magic.

Am I correct in thinking about the first line? and what's the next line doing?

Thanks

Answer 1

this.foo = bar assigns the value of bar (whatever it may be) to the object's member named foo . This can indeed be a function definition along with a host of other things.

What var trigger = t.trigger does is declare a variable named trigger in the scope of the EventManager function. There are other functions defined in a lower scope, namely pushLoading and popLoading . These are not object members, so using this.trigger inside of those functions would not work. The var trigger declaration makes it accessible in these functions.

I don't claim to be anywhere close to the best JS programmer ever, so I can't say whether what he's doing makes a ton of sense or not, but you can see how EventManager.js is actually used (in Calendar.js ):

EventManager.call(t, options, eventSources);
var isFetchNeeded = t.isFetchNeeded;
var fetchEvents = t.fetchEvents;

He does not actually create an EventManager object, but he calls its constructor with the Calendar object. Calender does have trigger defined, which is why var trigger = t.trigger works inside of EventManager . Similarly, the line

var isFetchNeeded = t.isFetchNeeded;

In Calendar.js acquires the isFetchNeeded method "back out of" EventManager and allows it to be called in the Calendar scope.

Answer 2

The variable this references the context the function was called in. In this case (assuming the code is being run in a web browser) is the window object. The first line simply assigns the function isFetchNeeded to be a method of the this object. The second line assigns the value of this 's member variable trigger to a variable local to the function EventManager .