原型链相关问题，求大佬不吝赐教？

Question

var A = function() {};
A.prototype.n = 1;
var a=new A()
A.prototype = {
  n: 2,
  m: 3
}
console.log(a.n);//1
console.log(a.m);//undefined

var A = function() {};
A.prototype.n = 1;

A.prototype = {
  n: 2,
  m: 3
}
var a=new A()
console.log(a.n);//2
console.log(a.m);//3

请问，为什么var a=new A()的位置会影响结果

Answer 1

var A = function() {};
A.prototype.n = 1;
var a=new A()
var p0 = A.prototype
console.log("a.__proto__ is p0", Object.is(Object.getPrototypeOf(a), p0))
A.prototype = {
  n: 2,
  m: 3
}
var p1 = A.prototype
console.log("p0 is p1", Object.is(p0, p1))
console.log("a.__proto__ is p0", Object.is(Object.getPrototypeOf(a), p0))
console.log("a.__proto__ is p1", Object.is(Object.getPrototypeOf(a), p1))
console.log("p0 is", p0)
console.log("p1 is", p1)
console.log(a.n);

a.__proto__ is p0 true
p0 is p1 false
a.__proto__ is p0 true
a.__proto__ is p1 false
p0 is { n: 1 }
p1 is { n: 2, m: 3 }
1

---

var A = function() {};
A.prototype.n = 1;
var p0 = A.prototype
A.prototype = {
  n: 2,
  m: 3
}
var p1 = A.prototype
var a=new A()
console.log("p0 is p1", Object.is(p0, p1))
console.log("a.__proto__ is p0", Object.is(Object.getPrototypeOf(a), p0))
console.log("a.__proto__ is p1", Object.is(Object.getPrototypeOf(a), p1))
console.log("p0 is", p0)
console.log("p1 is", p1)
console.log(a.n);

p0 is p1 false
a.__proto__ is p0 false
a.__proto__ is p1 true
p0 is { n: 1 }
p1 is { n: 2, m: 3 }
2

Answer 2

var A = function() {};
A.prototype.n = 1;

// 此时 A.prototype = {constructor: A, n: 1}

var a=new A()

// 此时 a.__proto__ = A.prototype = {constructor: A, n: 1}

A.prototype = {
n: 2,
m: 3
}

// 此时 A.prototype = {contructor: Object, n: 2, m: 3}，已经不是前面的原型对象了

这个时候再改 A 的 prototype 已经对 a 的 proto 没影响了， A 的 prototype 指向了新对象 {contructor: Object, n: 2, m: 3}， a 的 proto 指向的是 A 原来的 prototype 指向的对象 {constructor: A, n: 1}，所以 a.n 为 1。

把 var a=new A() 放在 A 改 prototype 之后时，a.__proto__ 指向的是改后的原型对象，所以 a.n 是 2。

Answer 3

因为是在new的时候确定继承关系的，new的时候构造函数的prototype指向什么，那就继承自谁