var testFn = function () {}testFn.prototype = {  fn1: function () {    console.log(this);    console.log('fn1');    return this;  },  fn2: function () {    console.log(this);    console.log('fn2');    return this;  },  fn3: function () {    console.log(this);    console.log('fn3');    return this;  }};var testFn = new testFn();testFn.fn1().fn2().fn3();

链式调用本质是: 实例化的函数调用原型对象上的方法；