"Uncaught TypeError: this._isDateType is not a function" within Arrow Function

Question

I've got every time a type error that a function definition could not be found. The code looks as follow:

return BaseController.extend("ch.micarna.weightprotocol.controller.Calendar", {
  onInit: function () {
    console.log(this._isDateType(new Date()));
    let oHbox = this.byId("calendar-container");
    let oTodayDate = new Date();
    let oEndDate = this._getLastDayOfMonth(oTodayDate);
  },

  _getLastDayOfMonth: (oBegin) => {
    if (this._isDateType(oBegin)) {
      throw new TypeError("The given parameter is not type of date.");
    }
    return new Date(oBegin.getFullYear(), oBegin.getMonth() + 1, 0);
  },

  _isDateType: (oDate) => {
    return Object.prototype.toString.call(oDate) === "[object Date]";
  },

});

The problem is the _isDateType function that could not be found when it is called inside the _getLastDayOfMonth function. I set the break point:

and as you can see, the function is undefined and I do not know why.

At the beginning of the onInit function, I called the _isDateType function:

console.log(this._isDateType(new Date()));

and it supply the result as expected.

What am I doing wrong?

Answer 1

Replace the arrow syntax ...

_getLastDayOfMonth: (oBegin) => {

... with a normal function expression:

_getLastDayOfMonth: function(oBegin) { // or _getLastDayOfMonth(oBegin) {
  // this === Controller instance
}

By this, _getLastDayOfMonth can freely access other methods within the Controller instance.

Why it didn't work with arrow function

• Arrow functions do not create their own this, which is why this usually refers to the context from the arrow function's lexical scope. Therefore, arrow functions, in general, should not be used as methods.

  Arrow functions don't have their own bindings to this, arguments, or super, and should not be used as methods. ^[source]

• In the above case, this was referring to the window object when BaseController.extend was called. Calling this._isDateType inside the arrow function was equivalent to window._isDateType.