function foo(a,b) {

    this.x = this.x + a + b;

}

//case 1:

var obj = {x:3};

foo.apply(obj,[1,2]);

//obj <==> {x:6}

//case 2:

var obj = {x:3};

foo.call(obj, 1, 2);

//obj <==> {x:6}

//case 3:

var obj = {x:3};

var foo1 = foo.bind(obj, 1, 2);

foo1();

//obj <==> {x:6}