Stub/Spies direct onto an Object

[quangv]

Hi, I'm new to BDD/TDD, Sinon.js is great btw :)

I'm using Sinon.js with Mocha & Node.js, and I was wondering if there was a way to add spies/stubs onto existing objects... so that any reference to that object now includes those spies... for example...

module.js

module.exports = function(){
}

subject.js

x = require('./module.js');
x()

test_subject.js

describe('subject', function(){
  it('should call x()', function(){
    x = require('./module.js');
    sinon.spy(x);
    require('./subject.js');
    assert.equal(x.called, true);  // failed
  });
})

might not be possible, thanks :)

just wanted to note that if module.js returns an object, it'll be spyable...

// module.js
exports.x = function(){
}

// subject.js
module = require('./module.js');
module.x();

// test_subject.js
module = require('./module.js');
sinon.spy(module, 'x');
require('./subject.js');
assert.equal(module.x.called, true);  // passes

related question for me, if X was instead a class, would it be possible? For example

// module.js
module.exports = function(){
}

// subject.js
X = require('./module');
new X;

// test_subject.js
X = require('./module');
sinon.spy(X, 'constructor');  // or sinon.spy(X.prototype, 'constructor'); ??
require('./subject');
assert.equal(X.calledWithNew(), true);

My understanding of how instances are created might be completely wrong. But is it correct that there is no way to turn an object into a spy, but instead have to create a new object, that's a spy? For example:

x = function(){};
sinon.spy(x);  // x isn't spied
y = sinon.spy(x)  // y is spied

x = {y:function(){}}
sinon.spy(x, 'y');  // x.y is spied, works; can the above work the same way?

Thank you :)

[neonstalwart]

your problem is related to how your subject.js module gets a reference to the exports of module.js.

subject.js gets a direct reference to the exports of module.js rather than a reference to the spy that you create based on the exports of that module.

when you export an object, subject.js gets a reference to that object but when it tries to access a property of that object, you've already replaced the original function attached to that property with the spy function - this is why it works when you export an object.

your problem is not so much related to what you can or cannot spy on but rather it is a problem of what your subject.js gets a reference to when it loads. there's probably a clearer explanation but maybe that helps you see what's happening.

[cjohansen]

As far as I can tell, there is no way to implement stubbing of functions that are directly exported (e.g module.exports = function () { ... }), because, like @neonstalwart said, you can replace the reference itself.

[funston]

why can't exporting a function reference work? it's perfectly valid JS and works in node.

this is valid:

var myObject = function () {};
module.exports = myObject;

[neonstalwart]

@rschiavi the issue is not whether or not it's valid. the problem is that there's no way to get hold of the exports (or module) from outside of the module, so you can't replace the exported function with a stubbed version.

[kris-at-tout]

What about when you define your export like this:

module.js
module.exports = {
x: function() { }
}

subject.js
y = require('./module.js');
y.x();

test_subject.js
describe('subject', function() {
it('should call x()', function() {
var x = require('./module.js');
sinon.spy(x)
require('./subject.js');
assert.equal(x.called, true);
})
})

module is exporting an object, not a function directly. The error message I get is:

) subject should call x():
TypeError: Attempted to wrap undefined property undefined as function
at Object.wrapMethod (/Users/kris/node_modules/sinon/lib/sinon.js:65:23)
at Object.spy (/Users/kris/node_modules/sinon/lib/sinon/spy.js:42:22)
at Context. (/Users/kris/Development/ntest/test_subject.js:6:11)
at Test.Runnable.run (/usr/local/lib/node_modules/mocha/lib/runnable.js:184:32)
at Runner.runTest (/usr/local/lib/node_modules/mocha/lib/runner.js:300:10)
at Runner.runTests.next (/usr/local/lib/node_modules/mocha/lib/runner.js:346:12)
at next (/usr/local/lib/node_modules/mocha/lib/runner.js:228:14)
at Runner.hooks (/usr/local/lib/node_modules/mocha/lib/runner.js:237:7)
at next (/usr/local/lib/node_modules/mocha/lib/runner.js:185:23)
at Runner.hook (/usr/local/lib/node_modules/mocha/lib/runner.js:205:5)
at process.startup.processNextTick.process._tickCallback (node.js:244:9)

[cjohansen]

@kris-at-tout You want to do:

sinon.spy(x, "x");

And:

x.x.called

That is, you stub the x property of the exported module from module.js