问题
Can anyone tell how the output became undefined?
var foo = {n: 2};
foo.x = foo = {n: 2};
console.log(foo.x); // undefined
回答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});
Graphical explanation
var foo = {n:2};
foo.x = foo = {n:2};
console.log(foo.x);
回答2:
I'll break down the assignment and output of each variable as it plays out.
var foo = {n:2}
# foo: Object {n: 2}
# foo.x: undefined
foo.x = 3
# foo: Object {n: 2, x: 3 }
# foo.x: 3
foo.x = foo
# foo: Object {n: 2, x: Object {n:2, x: Object (recursive) } }
# foo.x: Object {n: 2, x: Object {n:2, x: Object (recursive) } }
foo.x = foo = {n: 2}
# foo: Object {n: 2}
# foo.x: undefined
The last line, as you can see, resets foo to equal {n: 2} which makes foo.x non-existent (because you have overwritten the foo object)
回答3:
Your code is exactly the same as this one,
var foo = {n:2}; // foo ==> {n:2}
foo.x = {n:2}; // foo ==> {n:2,x:{n:2}}
foo = {n:2}; // foo ==> {n:2}
console.log(foo.x); // undefined
because there's no "x" key in your object
来源：https://stackoverflow.com/questions/37793432/double-assignment-of-an-object-property-results-in-undefined-property