Sinon Spy / Stub for Function inside Function (Private Function)

Question

I'm new Sinon. i'm unable to spy the private ajax function

Actual code in library.js

function ajax () {
    console.log("I'm a");
}

function getJSON() {
    ajax();
    console.log("I'm b");
}

exports.getJSON = getJSON;

Actual Test Code

var sinon = require('sinon');
var lib = require('./library');

describe('Tutor Test', function () {
    it('getJSON Calling ajax', function (done) {
        var ajax = sinon.spy(lib, 'ajax');
        lib.getJSON();

        ajax.restore();
        sinon.assert.calledOnce(ajax);
        done();
    });
});

Note: I already tried with below object example. it works like charm.

Working Code in library.js

var jquery = {
    ajax: function () {
        console.log("I'm a");
    },

    getJSON: function () {
        this.ajax();
        console.log("I'm b");
    }
};

exports.jquery = jquery;

Working Test Cases.

var sinon = require('sinon');
var $ = require('./library').jquery;

describe('Tutor Test', function () {
    it('getJSON Calling ajax', function (done) {
        var ajax = sinon.spy($, 'ajax');
        $.getJSON();

        ajax.restore();
        sinon.assert.calledOnce(ajax);
        done();
    });
});

I'm getting error like below during mocha test

1) Tutor Test getJSON Calling ajax:
     TypeError: Attempted to wrap undefined property ajax as function
      at Object.wrapMethod (node_modules/sinon/lib/sinon/util/core.js:113:29)
      at Object.spy (node_modules/sinon/lib/sinon/spy.js:41:26)
      at Context.<anonymous> (test.js:41:26)

Answer 1

As far as i know, Spying private Variables / Function is not possible in Sinon. So avoid using Sinon for those use cases.

Note: Your (ajax) function does not have param and return value and not bind to the exported object is real challenge for sinon

In such case, If you wanted to make sure whether ajax function is triggered or not. You can use rewire.

Below is the working code.

var rewire = require('rewire');
var lib = rewire('./library');
const assert = require('assert');

describe('Tutor Test', function () {
    it('getJSON Calling ajax', function (done) {
        lib.__set__('isAjaxCalled', false);
        lib.__set__('ajax', function () {
            lib.__set__('isAjaxCalled', true);
        });

        lib.getJSON();
        assert.equal(lib.__get__('isAjaxCalled'), true);
        done();
    });
});

No Change your Actual code, library.js