JavaScript - Owner of "this"

Solution 1:

If you want to have the this property be consistent, you should bind the functions that are being called.

For example,

setInterval(function() { /* code here */ }.bind(this), 500)

That way, the this of the inner function will be the same as that of the outer function.

Solution 2:

Whenever you see function you can assume the value of this changes, so inside the callback function for the interval this is actually the window, not the object.

The easy solution is to just store this in a variable

function Timer(){
    var time1 = null;
    var time2 = null;
    var timeLoop = null;

    function getTime(){
        var day = new Date();
        return day.getTime();
    }

    this.start = function(){

        var self = this;

        time1 = getTime();

        timeLoop = setInterval(function(){
            time2 = getTime();
            console.log(self.duration());
        },500);
    }

    this.duration = function(){
        return (time1 - time2) / 1000;
    }

}