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

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.