Side Effect in sinon.stub with returns().

[Rithwik Anand]

Stubs are showing an undesirable side effect when used with returns. Below code snippet shows the expected behavior (using actual function scenario) and stub's behavior.

var actualFunc = function () {return ["internalContent"];};
var stubFunc = sinon.stub().returns(["internalContent"]);

var actualFuncReturn = actualFunc();
var stubFuncReturn = stubFunc();

actualFuncReturn.push("sideEffect");
stubFuncReturn.push("sideEffect");

console.log(actualFunc()); //['internalContent'].
console.log(stubFunc());  // ['internalContent', 'sideEffect'].

Is this a bug? If no, how do you justify this behavior?

Thanks,

Rithwik.

[Maximilian Antoni]

Yes, there are these cases. We can't create a copy of the object because you might want to return a specific instance of something. The copy would lead to other unexpected cases like === not working anymore.

Another example of this is when a function modifies an array argument. If you spy on it and try to assert the passes in arguments, it doesn't work.

-Max

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[Rithwik Anand]

Thanks for the confirmation Max!

Since in my test case, avoiding side effects is more critical than doing equality checks, I do the following work around: 

var actualFunc = function () {return ["internalContent"];};

var stubFunc = sinon.stub();

var MAX_CALL_COUNT = 10; // Maximum invocations expected in current test case.
Array.apply(null, new Array(MAX_CALL_COUNT)).forEach(function (v, i) {
   stubFunc.onCall(i).returns(["internalContent"]);

});

var actualFuncReturn = actualFunc();
var stubFuncReturn = stubFunc();

actualFuncReturn.push("sideEffect");
stubFuncReturn.push("sideEffect");

console.log(actualFunc()); //['internalContent'].

console.log(stubFunc()); // ['internalContent'].

Do you suggest any better way of doing it?

Thanks,

Rithwik. 

[Maximilian Antoni]

You could also implement custom logic yourself. That would read nicer, I think:

  sinon.spy(function () { return [ ... ]; })

or with a stub:

  sinon.stub(object, 'func', function () { return [ ... ]; })

-Max