Unit Testing - Mock Methods within Methods?

I am writing unit tests for a Node.js application, and I am wondering if I am mocking the correct parts of the code.

The example below is a hypothetical class that has two static methods.

The method isTokenValid calls another method, decodeToken which takes the token and a callback. The callback is defined inside of isTokenValid. Both these methods belong to the same class.

When unit testing isTokenValid my approach is to mock the decodeToken method.

It is clear to me that when unit testing, dependencies such AJAX requests should be mocked. However, does that also hold true for this type of dependency or am I being too granular?

Is mocking decodeToken the right approach to unit testing isTokenValid?

var TokenClass = {};

TokenClass.isTokenValid(token) {
    TokenClass.decodeToken(token, function(err, decoded) {
            if (err) {
                console.log('There was a validation error');
            }
            if (decoded) {
                return true
            };
        }
    }

    TokenClass.decodeToken(token, callback) {
        // some logic here to decode token
        if (err) {
            return callback(err);
        }
        // if token is not valid
        if (!validToken) {
            return callback(null, undefined);
        }
        // if token is valid
        return callback(null, decoded);
    }
}