why the program's result is undefined?

Question

 var foo = {};
 foo.c = foo = {};
 console.log(foo.c);

why the result is undefined? i thought it is supposed to be \'[object Object]\'

Answer 1

Strange things are happening here in the assignments:

foo.c = (foo = {})

The reference to foo.c is resolved first and points to the old foo object, before the inner expression is evaluated where foo is re-assigned with the {} emtpy object literal. So your code is equivalent to

var foo1 = {};
var foo2 = {};
foo1.c = foo2;
console.log(foo2.c) // obviously undefined now

You can also try

var foo = {}, old = foo;
foo.c = foo = {};
console.log(old, foo, old.c===foo); // {c:{}}, {}, true

Answer 2

JavaScript engine splits such assignment:

a = b = c = 1;

As follow:

a = 1;
b = 1;
c = 1;

And not as:

c = 1;
b = c;
a = b;

There is a slightly but important difference – mostly, it involves getters, please check Multiple variable assignments in one row for further details – and that's mostly why your code doesn't behave like expected, because the initial expectations are based on false assumptions.