Accessing an object's property from an event listener call in Javascript

Solution 1:

When the event handler gets called, "this" no longer references the "someObj" object. You need to capture "this" into a local variable that the mouseMoving function will capture.

var someObj = function someObj(){
    this.prop = 33;
    var self = this;
    this.mouseMoving = function() { console.log(self.prop);}

    document.getElementById("someDiv").addEventListener('mousemove', this.mouseMoving, true);
}

I'm assuming "someObj is a constructor, i.e. intended to be called with as new someObj(), otherwise "this" will be the global scope.

The "this" keyword can be confusing in JavaScript, because it doesn't work the same way as in other languages. The key thing to remember is that it is bound to the calling object when the function is called, not when the function is created.

Solution 2:

The javascript built-in Function.prototype.bind() is intended for this purpose.
For example:

var someObj = function someObj(){
       this.prop = 33;
        this.mouseMoving = function() { console.log(this.prop);}

        document.getElementById("someDiv").addEventListener('mousemove', this.mouseMoving.bind(this),true);

 }

More on the bind method here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind

Other wise you have to pass a reference of the object someObj to the element and use that reference in the line:

console.log(this.referenceToObject.prop); //this references the DOM element in an event.

Solution 3:

From Section 4.3 of JavaScript: The Good Parts by Douglas Crockford:

Invoking a function suspends the execution of the current function, passing control and parameters to the new function. In addition to the declared parameters, every function receives two additional parameters: this and arguments. The this parameter is very important in object oriented programming, and its value is determined by the invocation pattern. There are four patterns of invocation in JavaScript: the method invocation pattern, the function invocation pattern, the constructor invocation pattern, and the apply invocation pattern. The patterns differ in how the bonus parameter this is initialized.

Crockford continues to explains the binding of 'this' in each of these patterns, as follows:

The Method Invocation Pattern: When a function is stored as a property of an object, we call it a method. When a method is invoked, this is bound to that object.

The Function Invocation Pattern: When a function is invoked with this pattern, this is bound to the global object. This was a mistake in the design of the language.

The Constructor Invocation Pattern: If a function is invoked with the new prefix, then a new object will be created with a hidden link to the value of the function's prototype member, and this will be bound to that new object.

The Apply Invocation Pattern: The apply method lets us construct an array of arguments to use to invoke a function. It also lets us choose the value of this. The apply method takes two parameters. The first is the value that should be bound to this. The second is an array of parameters.