function foo() {
  console.log( this.a );
}
var obj = {
  a: 2,
  foo: foo
};
obj.foo(); // 2

function baz() {
  // call-stack is: `baz`
  // so, our call-site is in the global scope

  console.log( "baz" );
  bar(); // <-- call-site for `bar`
}

function bar() {
  // call-stack is: `baz` -> `bar`
  // so, our call-site is in `baz`
  console.log( "bar" );
  foo(); // <-- call-site for `foo`
}
function foo() {
  // call-stack is: `baz` -> `bar` -> `foo`
  // so, our call-site is in `bar`
  console.log( "foo" );
}
baz(); // <-- call-site for `baz`

function foo() {
  console.log( this.a );
}
var obj2 = {
  a: 42,
  foo: foo
};
var obj1 = {
  a: 2,
  obj2: obj2
};
obj1.obj2.foo(); // 42
Implicitly Lost（隱式丟失）
function foo() {
  console.log( this.a );
}
var obj = {
  a: 2,
  foo: foo
};
var bar = obj.foo; // function reference/alias!
var a = "oops, global"; // `a` also property on global object
bar(); // "oops, global"

function foo() {
  console.log( this.a );
}
function doFoo(fn) {
  // `fn` is just another reference to `foo`
  fn(); // <-- call-site!
}
var obj = {
  a: 2,
  foo: foo
};
var a = "oops, global"; // `a` also property on global object
doFoo( obj.foo ); // "oops, global"