Unable to spy on existing function

Question

I'm unable to spy on an existing function in the current scope within node.js :

function myFunc() {console.log("Spy on me and I'll have you arrested"};
sinon.spy(myFunc);
myFunc.restore(); // throws an error: myFunc.restore is not a function

I can however spy on a function which is a member of an object:

var anObject = {
  myFunc: function() {console.log('Being spied on turns me on');}
};
sinon.spy(anObject, 'myFunc');
sinon.myFunc.restore(); // smooth sailing

According to the docs , it seems to me like that should work fine. How do I get this done?

Answer 1

In JavaScript when a function is passed as an argument it is a reference-passed-by-value, like so:

function foo() { console.log("foo"); } // func1, referenced by `foo`
function bar() { console.log("bar"); } // func2, referenced by `bar`

function mutate(func) {
    func = bar;
}

mutate( foo );
foo();

This will print out "foo" , not "bar" , because mutatate does not change foo 's reference to func1 .

Here is the relevant source code for Sinon's spy.js : https://github.com/sinonjs/sinon/blob/master/lib/sinon/spy.js

The create function sees if the first argument is a function, and if so, it wraps it in a proxy ( create: function create(func, spyLength) { , line 148). It then returns the proxy.

So in your case, you need to replace myFunc with the new proxy:

function myFunc() {console.log("Spy on me and I'll have you arrested"};
myFunc = sinon.spy(myFunc); // here

However you cannot use myFunc.restore() to undo the spy because .restore cannot change the target of the myFunc reference. Note that restore also does not return a value, so you must keep track of myFunc yourself:

function myFuncOriginal() {console.log("Spy on me and I'll have you arrested"};
var myFunc = sinon.spy(myFuncOriginal);
myFunc = myFuncOriginal; // instead of `myFunc.restore();`