在函数返回之前访问它自己的对象属性

Question

In the below code, the anonymous function run by the forEach loop is able to call subscription.dispose() on its first run-through, before the buttonClicks.forEach(..) has returned the subscription object to the subscription variable.

How can forEach(..) have access to an object it returns, before forEach has finished running at least once?

function(button) {
    var buttonClicks = Observable.fromEvent(button, "click");

    // In the case of an Observable, forEach returns a subscription object.
    var subscription =
        buttonClicks.
            forEach(function(clickEvent) {
                alert("Button was clicked. Stopping Traversal.");

                // Stop traversing the button clicks
                subscription.dispose();
            });
}

Answer 1

The variable subscription is already declared.

It will be initialized after forEach returns, before that, it is undefined .

So the click handler can refer to the variable. What it cannot do is actually call dispose , for that it needs that variable to hold a value (but this won't happen until someone clicks, and by then the forEach has long returned and the variable assigned).

Note that Observable#forEach does not immediately run the function you pass it (unlike, say, Array#forEach ). The function is run every time the observable emits a value (here: a click has happened).

Also note that you could later assign subscription to something else, and it will call dispose on that. It is not really tied to the object returned by forEach . The click handler will continue to refer to the variable , not to the value of the variable at the time it was created.