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

the code is as follow:

var pattern = {
get: function () {
    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();

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