js function test if it is called by new operator

在javascript function中，如果要強制使用new，可以參考以下nodejs cluster/worker.js的範例

他這邊利用檢查this是否是instanceof Worker，在一般情況下

• https://www.w3schools.com/js/js_this.asp
  • In a function, this refers to the global object.
  • In a function, in strict mode, this is undefined.

但是在new operator下

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new

The new keyword does the following things:
1. Creates a blank, plain JavaScript object;
2. Links (sets the constructor of) this object to another object;
3. Passes the newly created object from Step 1 as the this context;
4. Returns this if the function doesn’t return its own object.