JavaScript code trick: What’s the value of foo.x

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

determines that foo.x refers to a property x of the {n: 1} object, assigns {n: 2} to foo, and assigns the new value of foo – {n: 2} – to the property x of the {n: 1} object.

The important thing is that the foo that foo.x refers to is determined before foo changes.

See section 11.13.1 of the ES5 spec:

  1. Let lref be the result of evaluating LeftHandSideExpression.

  2. Let rref be the result of evaluating AssignmentExpression.

The assignment operator associates right to left, so you get:

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

The left hand side is evaluated before the right hand side.

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