如何理解下面一段js代码？

Question

vara={"x":1};varb=a;console.log(b.x);a.x=2;console.log(b.x);a={"x":3};console.log(b.x);a.x=4;console.log(b.x);为什么输出的是1222

Answer 1

vara={"x":1};//a:object#1{"x":1}
varb=a;//b:object#1{"x":1}
console.log(b.x);//1
a.x=2;//a,b:object#1{"x":2}
console.log(b.x);//2
a={"x":3};//a:object#2{"x":3}
console.log(b.x);//2
a.x=4;//a:object#2{"x":4}
console.log(b.x);//2