这就是原型链的作用。

function User(uid, pwd) {
  this.uid = uid
  this.pwd = pwd
}

User.prototype.displayAll = function() {
  document.write(this.uid)
  document.write(this.pwd)
}


var aaron = new User("Aaron", "123");


aaron.displayAll();

另一种方法是使用Class语法。

class User {
  constructor(uid, pwd) {
    this.uid = uid;
    this.pwd = pwd;
  }

displayAll(){
    document.write(this.uid);
    document.write(this.pwd);
  }
}

var Aaron = new User("Aaron", "123");
Aaron.displayAll();

您可以从function displayAll()更改为this.displayAll = function displayAll()

function user(uid, pwd)
    {
        this.uid = uid
        this.pwd = pwd
        this.displayAll = function displayAll()
        {
            document.write(uid)
            document.write(pwd)
        }
    }

    var Aaron = new user("Aaron", "123")

    document.write(Aaron.uid)
    Aaron.displayAll();