Side effects of stubbing console in tests

4 minute read

We all love stubbing objects in unit test as it makes our life easier when writing tests. You can stub objects created by your source code or objects which our environment specific like node or browser. On ESLint project, we were stubbing console object so that we can see a clean output for the test runs. But later on we figured out that if the test has some errors then it will not be displayed by the testing framework on the cmd because we have stubbed the console object completely.

For better understanding of stubbing please reference excellent article by Elijah Manor - Unit Test like a Secret Agent with Sinon.js.

Lets run through some scenarios of what happens when you do mock console vs when you don’t mock console. We will be using the following libraries1 for demonstration purposes:

  1. Mocha
  2. Sinon.JS
  3. Chai asserts
  4. Proxyquire

Scenarios

Test pass and console is not mocked

Without mocking anything, if we run a sample test with all test passing.

var assert = require("chai").assert;
var sinon = require("sinon");

describe("validation", function(){
    it("Expect it to be true", function(){
        assert.isTrue(true);
    });

    it("Expect it to be false", function(){
        assert.isFalse(false);
    });
});

The output will be

$ ./node_modules/.bin/mocha test.js

  validation
    √ Expect it to be true
    √ Expect it to be false


  2 passing (6ms)

Everything works as expected and mocha is able to easily display the output for the test.

Test fail and console is not mocked

Without mocking anything, if we run a sample test in which one test has failure.

var assert = require("chai").assert;
var sinon = require("sinon");

describe("validation", function(){
    it("Expect it to be true", function(){
        assert.isTrue(false);
    });

    it("Expect it to be false", function(){
        assert.isFalse(false);
    });
});

The output will be

$ ./node_modules/.bin/mocha test.js

  validation
    1) Expect it to be true
    √ Expect it to be false


  1 passing (8ms)
  1 failing

  1) validation Expect it to be true:
     AssertionError: expected false to be true
      stack trace...

Everything works as expected and mocha is able to easily display the output for the test failure and the stack trace for information about the error.

Test fail and console is mocked

We will mock some common methods on console object and then the test which has some failures.

var assert = require("chai").assert;
var sinon = require("sinon");

describe("validation", function(){
    beforeEach(function(){
        this.cStub1 = sinon.stub(console, "info");
        this.cStub2 = sinon.stub(console, "log");
        this.cStub3 = sinon.stub(console, "error");
        this.cStub4 = sinon.stub(console, "trace");
    });
    afterEach(function(){
        this.cStub1.restore();
        this.cStub2.restore();
        this.cStub3.restore();
        this.cStub4.restore();
    });

    it("Expect it to be true", function(){
        assert.isTrue(false);
    });

    it("Expect it to be false", function(){
        assert.isFalse(false);
    });
});

The output will be

$ ./node_modules/.bin/mocha test.js

  validation

  1 passing (13ms)
  1 failing

  1) validation Expect it to be true:
     AssertionError: expected false to be true
      stack trace...

Everything works as expected and mocha is able to easily display the output for the test failure. But if you see closely the individual test run checks didn’t get printed. Now that is because your test stubbed the console object. Since this is very simple example that is why the damage is minimum but it can lead to more bigger issues.

Worst scenario

when you do all the mocking inside the test (inside it) and not inside beforeEach. Then if a test fails the stub never restores itself.

var assert = require("chai").assert;
var sinon = require("sinon");

describe("validation", function(){
    it("Expect it to be true", function(){
        this.cStub1 = sinon.stub(console, "info");
        this.cStub2 = sinon.stub(console, "log");
        this.cStub3 = sinon.stub(console, "error");
        this.cStub4 = sinon.stub(console, "trace");
        
        assert.isTrue(false);
        
        this.cStub1.restore();
        this.cStub2.restore();
        this.cStub3.restore();
        this.cStub4.restore();
    });

    it("Expect it to be false", function(){
        assert.isFalse(false);
    });
});

The output will be

$ ./node_modules/.bin/mocha test.js

  validation

Here nothing gets displayed because console never got restored to its original state. If you have many tests then this can lead to potential undetected issues. Debugging is a nightmare in this situation.

Solution

Best way to tackle such scenarios is to extract the logging logic into its own module and then stub that module inside your tests.

Create logging.js module2 to be consumed.

module.exports = {
    info: function() {
        console.log.apply(console, Array.prototype.slice.call(arguments));
    },

    error: function() {
        console.error.apply(console, Array.prototype.slice.call(arguments));
    }
};

Consume logging module created above inside your app.js module.

var log = require("./logging.js");

module.exports = {
    sayHi: function(name){
        log.info("Hi " + name);
    }
};

So this way when you write unit test for the app module you can actually stub logging module.

var assert = require("chai").assert;
var sinon = require("sinon");
var proxyquire = require("proxyquire");

proxyquire = proxyquire.noCallThru().noPreserveCache();

describe("validation", function(){
    var log = {
        info: sinon.spy(),
        error: sinon.spy()
    };

    it("Expect it to be true", function(){
        var app = proxyquire("../lib/app", {
            "./logging": log
        });
        app.sayHi("test");
        assert.isTrue(log.info.calledOnce);
    });

    it("Expect it to be false", function(){
        assert.isFalse(false);
    });
});

Conclusion

Always avoid stubbing console as there is always a better way to achieve what you are trying to do. Sometimes stubbing like this may lead to issue where you will not know that there are issues since they all are silently getting ignored.

References
  1. For list of libraries used please reference the top section of the article. 

  2. Some of the code examples have been taken from ESLint library. 

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