How to properly test such constructs using Mocha, Chai, Sinon?

Question

Node.js

Hello dear community. I ask you to suggest/indicate/direct to the true path). Just started learning nodejs and trying to start writing tests. And almost immediately the question arose:
There is, let's say, such a design, simplified:

method1(arg1, arg2, callback);

method2(arg3, arg4, callback);

method3(arg5, callback) {
  async.waterfall([
    function(cb) {
      method1(agr1, arg2, function(error, result1) {
        if (error) return cb(error);
        
        // ... smth code
        
        cb(null, result1);
      });
    },
    function(result1, cb) {
      method2(agr3, arg4, function(error, result2) {
        if (error) return cb(error);
        
        // ... smth code
        
        cb(null, result2);
      });
    }
  ], function(error, results) {
    
    // ... smth code
    
  });
});

The questions are, in fact, the following:
1) Is it worth testing method1, method2 separately? Or is it possible to test them already for a specific method3?
2) I read that sinon is used for a more concise type of tests, but even here the question arises, because it is not clear what to grab onto?
If I understand the general approach of writing tests correctly, it is desirable to wrap each method/function in a separate test. If some third-party things are used in one method, then wrap them in spy, stub, mock. But after a considerable amount of time, I still can’t find anything worthwhile specifically for this example.
Thanks in advance!

Answer 1

If you do not call method1 and method2 from outside, then you should not cover them with tests, because even if they disappear, the external code will not know about it.