js es6之前如何实现继承

function Person(){}
Person.prototype.dance = function(){};

function Ninja(){}
Ninja.prototype = new Person();

//因为上方设置原型，导致了constructor也被指向了Person，手动改回来
Object.defineProperty(Ninja.prototype,"constructor",{
  enumerable:false,//不可遍历
  value:Ninja,
  writable:true
}