Baidu HR to give the JS face questions

Say the following code to run the results

var a = {N:1}
var B = A;
a.x = a = {N:2}
Console.log (a.x);
Console.log (b.x)


Output:
Undefined
N:2


parsing:
var a = {N:1}//defines variable A as a new object, and the new object has an n member with a value of 1
var B = A; Define variable B, and let B also refer to the object of variable a
After these two sentences, both variables A and B refer to the object {N:1}

The following sentence is the key to the problem
a.x = a = {N:2}
A.x = Represents the x member of the object {N:1} to which a reference is to be assigned
In fact, variable B also references this object {N:1}
At this point, the JavaScript engine first adds an empty x member to {N:1}, namely: {n:1,x:undefined}
What is the value of the next assignment to {n:1,x:undefined}.x?
The value to assign is a = {N:2}
That is, let variable a refer to a new object {N:2} first
And then assign the new value of a to {n:1,x:undefined}.x
So the original object becomes the {N:1,x:{n:2}}
Because variable b always references that original object, the value of B is {N:1,x:{n:2}}
Because variable A is given a new object, the value of a is {N:2}

These two lines are console output.
Console.log (a.x); The object referenced by variable A is {N:2}, and there is no X member, so the output undefined
Console.log (b.x); The object referenced by variable b is {N:1,x:{n:2}}, so the output {N:2}

Web front-end course tools and e-books are required, plus 22 groups of 120342833