TDD (Test-Driven-Development) is getting popular day by day defining its needs in greater code testing. This process is a test which we define to get an output by applying to our library, API etc. Day by day Nodejs is now developing as a language of choice for REST API development, and Express has become as a framework of choice.

The first step is to test your APIs you create before building an Express project. lets see how:

STEP 1: Here, we are starting with a simple Express application that provide API functionality for addition and subtraction

• Run following in a terminal to download example application-

git clone https://github.com/cianclarke/tdd-for-apis.git ; cd tdd-for-apis # Clone the finished repository
git checkout boilerplate # jump to the initial project code

• Now, download dependencies that will be required further development process. Run following commands to set up test framework.

npm install --save-dev supertest
npm install --save-dev mocha

Once above step is done, we will have our testing framework ready. Now the next process to create our first integration test.

STEP 2: Creating Integration test

• Create a directory in which you will take test a nd blank file to store test file. For that first, run following commands form root directory

mkdir -p test/integration
touch test/integration/test-route-addition.js

• let ‘s, we will write an integration test, for that we have mount our route at /add and it will take 2 query string params called a and b. Our Expected output status should be 200(OK), if any values get missed or is non-integer value, then we receive status code 422 un-processable entity.

Our test will look like:

var supertest = require('supertest'),
app = require('../../app');

exports.addition_should_accept_numbers = function(done){
  supertest(app)
  .get('/add?a=1&b=1')
  .expect(200)
  .end(done);
};

exports.addition_should_reject_strings = function(done){
  supertest(app)
  .get('/add?a=string&b=2')
  .expect(422)
  .end(done);
};

• Run following command to run the subtraction test case;

touch test/integration/test-route-subtraction.js

• Our new code will be:

var supertest = require('supertest'),
assert = require('assert'),
app = require('../../app');

exports.addition_should_respond_with_a_numeric_result = function(done){
  supertest(app)
  .get('/subtract?a=5&b=4')
  .expect(200)
  .end(function(err, response){
    assert.ok(!err);
    assert.ok(typeof response.body.result === 'number');
    return done();
  });
};

STEP 3: Run the integration tests

• In the downloaded code, You can find the files /add and /subtract which are already mounted in app.js, their defining routes can be found in /routes directory. The 200(OK) status code will be returned by these routes, this provides starting functionality-creating failing test, then implement.

Run following command:

node_modules/.bin/mocha -u exports test/integration/*

 You can see all the changes for this step, including what npm does when we run `install --save-dev` on GitHub.

STEP 4:  Make integration tests pass

• Now Let’s implement two routes, /addition and /subtraction, this will take number and return a the result in numeric form.
• Create lib directory with two files add.js and subtract.js, both of them will export function that will return 0 .Run the following commands:

mkdir lib
echo "module.exports = function(){ return 0; };" > lib/add.js
echo "module.exports = function(){ return 0; };" > lib/subtract.js

• Set up route handlers to use add and subtract function. In routes/add.js :

var add = require('../lib/add');
module.exports = function(req, res, next){
  return res.json({ result : add(req.a, req.b) });
};

• Update app.js :

app.use(function(req, res, next){
  var a = parseInt(req.query.a),
  b = parseInt(req.query.b);
  if (!a || !b || isNaN(a) || isNaN(b)){
    return res.status(422).end("You must specify two numbers as query params, A and B");
  }
  req.a = a;
  req.b = b;
  return next();
});

STEP 5: Add unit tests

• Add unit tests to check if our addition and subtraction logic fails. Let ‘s create blank files for our addition and subtraction. Run following commands to create test files:

mkdir test/unit
touch test/unit/test-lib-addition.js ; touch test/unit/test-lib-subtraction.js;

• Addition test will look like this:

var assert = require('assert'),
add = require('../../lib/add');

exports.it_should_add_two_numbers = function(done){
  var result = add(1,1);
  assert.ok(result === 2);
  return done();
};

exports.it_should_add_two_negative_numbers = function(done){
  var result = add(-2,-2);
  assert.ok(result === -4);
  return done();
};

STEP 6: How to run tests by users:

• Add an entry to our package.json file like,

"scripts" : {
    "test" : "node_modules/.bin/mocha -u exports test/**/*"
  }

• Run our test by following command and it will show result :

 npm test