Double-assignment of an object property results in undefined property [duplicate]

Answer 1

foo.x = foo = {n:2};

The foo.x refers to the property x to the object refered to by foo. However, foo = {n:2} assigns a completely new object to foo. x is indeed assigned to an object, but that object is immediately replaced by another object. The object with the x property isn’t referenced by anything anymore.

You can read that line as

foo.x = (foo = {n:2});

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Graphical explanation

var foo = {n:2};

foo.x = foo = {n:2};

console.log(foo.x);