引用mdn
对象的创建就像调用了new object（）一样；也就是说，由对象文字表达式生成的对象是对象的示例。
那么，在你的代码中 obj “自动”成为 Object .
分配 Object.prototype.a 在您的代码中将创建 a 作为一切继承物的财产 Object （在js中，即：所有内容）随后创建。

Object.prototype.a = 'I am the one';
let obj = {};
let Constrctr = function(aa = this.a) {
  //                          ^ QED
  this.aa = aa;

};
let obj2 = new Constrctr();
let obj3 = new Constrctr(2);
let someArray = new Array();
let someArrayLiteral = [];
let someNumber = new Number();
let someNumberLiteral = 42;
let someRegex = new RegExp();
let someString = new String();
let someStringLiteral = "";
let someFunction = () => {};

document.querySelector('pre').innerHTML = `<h2><i>Everything</i></h2>obj.a: ${obj.a},
obj2.aa: ${obj2.aa}, 
obj2.a: ${obj2.a},
obj3.aa: ${obj3.aa},
obj3.a: ${obj3.a},
someNumber.a: ${someNumber.a},
someNumberLiteral.a: ${someNumberLiteral.a},
someArray.a: ${someArray.a},
someArrayLiteral.a: ${someArray.a},
someRegex.a: ${someRegex.a},
someString.a: ${someString.a},
someStringLiteral.a: ${someStringLiteral.a},
someFunction.a: ${someFunction.a}`;

<pre></pre>