Using default parameter problem

I assign a default value to the function parameter and I cannnot run Ionic app on iOS.

like this:
fn: function(param1 = 1) { ...}
or
function fn(param1 = 1) { ... }

It works well with 'ionic serve' but not with emulator nor real device.

My app shows splash screen only.

After removing default parameters, app runs well.

Is not able to use default parameters?