javascript passing an object into function [duplicate]

Question

var o = {};
(function(x){x=null})(o);       //o is NOT null after this statement
(function(x){x.foo = "foo"; x.bar = "bar";})(o)   //o has properties foo and bar after this statement

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what is going on when passing object o into the function? the first function makes it seem o did not get passed; the second function makes it seem that o get passed

Answer 1

Line 1: An object is created. A reference to it is passed to o.

Line 2: A function is called. A reference to the object is passed as an argument. The reference (in x) is then overwritten with null (without touching the object itself or the reference to it still assigned to o).

Line 3: A function is called. A reference to the object is passed as an argument. A foo and bar property are added to the object and assigned values.