In javascript,how to understand the keyword-- "this"?

the code is as follow:

var pattern = {
get: function () {
    alert(this===instance)//true
    return 'I alway return this string,whatever you have assigned';
},
set: function () {
    this.myname = 'this is my name string';
}
};


function TestDefineSetAndGet() {
Object.defineProperty(this, 'myproperty', pattern);
}

var instance = new TestDefineSetAndGet();
alert(instance.myproperty);

my question is why the "this" in pattern.get refers to the object "instance"?