Cannot stub private methods in native JavaScript classes about sinon

[fatso83]

This is not a bug. To quote MDN:

Private properties get created by using a hash # prefix and cannot be legally referenced outside of the class. The privacy encapsulation of these class properties is enforced by JavaScript itself.

Sinon cannot do magic: if the JS engine itself prevents any other object from reaching the property, neither can we. As you can see from the below screenshot, there is no trace of the private field in the MyClass prototype. If it does not have a property for us to manipulate, we cannot reference it, hence the quite correct "Cannot stub non-existent property #aPrivateMethod", as there is no such property.

And frankly, that is exactly how it should be. If your unit tests are tampering with privates you should rightly be prevented from accessing them 😄 If you don't want JS to enforce privacy, don't use the feature. You could then use a convention as in the old days of having __privateProperty or something, if you really wish to.