Accessing this in anonymous function

Question

I want to create a prototype function that has its own scope. For this, I use an anonymous function but I cannot find a way to access the members of the object.

Here is a simplified version of what I am trying to achieve:

function F() {
    this.counter = 0;
} 

F.prototype.increment = (function() {
    var lastIncrementTime = -1;
    var caller = this; // <--- it fails here because this is the Window object
    return function(time) {
        if (time > lastIncrementTime) {
            caller.counter++;
            lastIncrementTime = time;
            return caller.counter;
        }
        return caller.counter;
    }
})();

f = new F();

f.increment();

I know it fails because this does not refer to F or the f object.

Is there a way to access it?

Answer 1

The immediately invoked function expression (IIFE) itself only gets invoked once, all calls to increment will use the variables as they were last left and not re- var them.

Change the invocation context using call , apply or bind

F.prototype.increment = (function() {
    // this === F.prototype
    // ...
}).call(F.prototype);

The this in this example context will not be instance specific, but be the prototype.

---

It seems like you actually want to achieve something a little bit different, where you have an independent function to initialise an instance-specific property with it's own closure, when the instance is constructed. These types of actions can consume a bit of memory so don't store too much unique data.

function F() {
    this.counter = 0;
    this.__init_increment(); // create `this.increment`
}
F.prototype.__init_increment = function () {
    var lastIncrementTime = -1;
    this.increment = function (time) {
        if (time > lastIncrementTime) {
            this.counter++;
            lastIncrementTime = time;
        }
        return this.counter;
    };
};
var f = new F();
f.increment(0); // 1
f.increment(0); // 1
f.increment(5); // 2

In this example, this.increment is a different function for each instance, which means you have a different closure for each instance. They are generated by a function in the prototype , which sets the instance property. The generator does not have to be in the prototype, just remember about the invocation context when applying it to your instance.

Answer 2

将var caller = this移动到匿名函数中， this将在适当的位置设置。