That strange syntax with the arrow function is only needed so that the 'this' inside the function behaves the way one would expect it to in normal OOP language.

That strange syntax with the arrow function is only needed so that the 'this' inside the function behaves the way one would expect it to in normal OOP language. It 'captures' the this for the object itself (like C++ would, etc). But that 'build()' function is what constructs a string that is the content for the dialog.