在此模块外部的Node.js模块中模拟构造函数（或其他函数）

Question

I'm using Jasmine to write my tests, but I guess I'd have this problem with any other testing framework. Let's say that we have a module foo with two functions, Bar and Baz , which are constructors (but could be just normal functions):

var Bar = exports.Bar = function Bar() {
  this.baz = new Baz();
};

var Baz = exports.Baz = function Baz() {

};

Now I would like to test Bar , but with a fake Baz implementation:

var foo = require('foo');

describe("foo.Bar", function() {
  it("initializes its own Baz", function() {
    spyOn(foo, 'Baz'); // this replaces foo.Baz with a fake implementation
    var bar = new foo.Bar();
    expect(foo.Baz).toHaveBeenCalled();
  });
});

The problem is that this test will fail, because Bar instantiates a new Baz using the variable Baz , which cannot be changed from outside. The only thing that got swapped by using spyOn() is exports.Baz .

The obvious solution is to write this.baz = new exports.Baz(); but it kind of feels awkward. If I have more functions which I want to use inside my module, I would have to always call all of them using the exports. prefix. Is there any other approach here?

Answer 1

If you can somehow decouple these two classes, like allow other implementation of the Baz class to be given to bar, then I think that's the best way and you should go for it.

---

But if you really want to be able to be able to refer to exports.Baz as Baz , then there's one way that I could think of, using with .

It is said that using with is generally a bad practice and should be avoided, I wouldn't use it in my own code, but this is one way to solve it and can even be a legimate use , as long as you know what you are doing.

Here it goes:

with(exports) {

    exports.Bar = function Bar() {
        console.log('this is bar!');
        this.baz = new Baz();
    };

    exports.Baz = function Baz() {
        console.log('original baz!');
    };

}

In another module if you change foo.Baz to something else, Baz inside foo will also look up to it.

I would still recommend finding a way to make these two classes independent of each other and then you can just give Bar whatever implementation of Baz you want.