Extend the prebuilt function addShape of Toast UI Image Editor

Question

I want to understand the approach if I want to extend the prebuilt function of Toast UI Image Editor javascript library. In this library there is function addShape(link below) which can be used to draw three already defined shapes i.e. Rectangle, Circle, Triangle. I want to extend the functionality of this function to incorporate another shape say SemiCircle. I have written code to draw semicircle but how I can add it to addShape function. Currently I have developed a new function with name addNewShape and handle the incoming request accordingly but I want to use the overriding nature of inhertance.

Function.prototype.inherits = function(parent) {
    this.super_ = parent;
    this.prototype = Object.create(parent.prototype);
  };

  myImageEditor.inherits(tui.ImageEditor); //Inherits all properties and methods

  function myImageEditor() {
    tui.ImageEditor.apply(this, arguments);
  }

myImageEditor.prototype.addNewShape = function (type, options) {
    if (type == 'semiCircle') {
        semiCircle(radius,x,y)
        } else {
            this.addShape(type, options); //for rect,circle and triangle
        }
    }



var imageEditor = new myImageEditor(document.querySelector('#tui-image-editor'), {

   cssMaxWidth: 700,
   cssMaxHeight: 500,
   selectionStyle: {
       cornerSize: 20,
       rotatingPointOffset: 70
   }
});

How I can override the addShape method so that for semicircle my function should be called otherwise default function?

I do not want to change the library itself, only solution to extend the existing functionality.

Toast UI Image Editor :https://ui.toast.com/tui-image-editor/ Function : https://nhn.github.io/tui.image-editor/latest/ImageEditor#addShape