How to unit test private methods in Typescript

Technically, in current versions of TypeScript private methods are only compile-time checked to be private - so you can call them.

class Example {
    public publicMethod() {
        return 'public';
    }

    private privateMethod() {
        return 'private';
    }
}

const example = new Example();

console.log(example.publicMethod()); // 'public'
console.log(example.privateMethod()); // 'private'

I mention this only because you asked how to do it, and that is how you could do it.

Correct Answer

However, that private method must be called by some other method... otherwise it isn't called at all. If you test the behaviour of that other method, you will cover the private method in the context it is used.

If you specifically test private methods, your tests will become tightly coupled to the implementation details (i.e. a good test wouldn't need to be changed if you refactored the implementation).

Disclaimer

If you still test it at the private method level, the compiler might in the future change and make the test fail (i.e. if the compiler made the method "properly" private, or if a future version of ECMAScript added visibility keywords, etc).

A possible solution to omit Typescript checks is to access the property dynamically (Not telling wether its good).

myClass['privateProp'] or for methods: myClass['privateMethod']()