Calling original function from Sinon.js Stub

Question

I'm trying to intercept a call with Sinon.js so I can do some logging and then execute the original call. I don't see a way to do this with sinon.spy(), but I think I can do it with sinon.stub().

I provided a custom function:

sinon.stub(servicecore.ServiceWrapper.prototype, '_invoke', function(method, name, body, headers, callback) {
    console.log('---- ServiceWrapper._invoke called! ----');

// How do I call the original function?

});

The problem I have is executing the original function, so my application behaves the same. Any idea?

Answer 1

You could use a closure. For example:

var obj = {
    foo: function () {
        console.log('foo');
    }
};

var stub = (function () {
    var originalFoo = obj.foo;
    return sinon.stub(obj, 'foo', function () {
        console.log('stub');
        originalFoo();
    });
}());

JSFiddle

Answer 2

Sinon stores a reference to the original function in the wrappedMethod property of the stub (docs were just recently added in 2020). This can be called in the fake method.

sinon.stub(Array.prototype, 'sort').callsFake(
  // Don't use arrow function => syntax if you need to use 'this' below!
  function () {
    console.log(`sorting array ${this}`);
    return Array.prototype.sort.wrappedMethod.apply(this, arguments);
  }
);

const array = ['C', 'A', 'B'].sort();
console.log(`sorted array is ${array}`);

<script src="https://cdnjs.cloudflare.com/ajax/libs/sinon.js/7.3.2/sinon.min.js"></script>

And so the OP's code would be:

sinon.stub(servicecore.ServiceWrapper.prototype, '_invoke').callsFake(function(method, name, body, headers, callback) {
    console.log('---- ServiceWrapper._invoke called! ----');
    return servicecore.ServiceWrapper.prototype._invoke.wrappedMethod.apply(this, arguments);
});

Answer 3

You can access the original method stored the in stub using the wrappedMethod property.

It is useful to reference the stub using a closure, avoid a messy this context, and allowing you to use a lambda function for the fake callback.

const stub = sinon.stub(patchSync, 'patchRemote').callsFake((...args) => {
  console.log('before call')
  const result =  stub.wrappedMethod(...args)
  console.log('after call')
  return result
})