What is a stub in Node.js ?

Last Updated : 19 Aug, 2020

A small program routine that substitutes for a longer program which is possible to be loaded later or that is remotely located.

Features of stub:

Stubs can be either anonymous.
Stubs can be wrapped into existing functions. When we wrap a stub into the existing function the original function is not called.
Stubs are functions or programs that affect the behavior of components or modules.
Stubs are dummy objects for testing.
Stubs implement a pre-programmed response.

Example:

javascript

var fs = require('fs')
var writeFileStub = sinon.stub(fs, 
    'writeFile', function (path, data, cb) {  
        return cb(null)
})

expect(writeFileStub).to.be.called  
writeFileStub.restore()

When to use stubs?

Prevent a specific method from being called directly.
Controlling method behavior down a specific path from a test to force the code. For example: Error handling.
Replacing the problematic pieces of code.
Testing asynchronous code easy.

Example To Create Asynchronous Stub That Throws An Exception:

javascript

require("@fatso83/mini-mocha").install();

const sinon = require("sinon");
const PubSub = require("pubsub-js");
const referee = require("@sinonjs/referee");
const assert = referee.assert;

describe("PubSub", function() {
   it("Calling all the subscribers, 
     irrespective of exceptions.", function() {
       const message = "an example message";
       const stub = sinon.stub().throws();
       const spy1 = sinon.spy();
       const spy2 = sinon.spy();
       const clock = sinon.useFakeTimers();

       PubSub.subscribe(message, stub);
       PubSub.subscribe(message, spy1);
       PubSub.subscribe(message, spy2);

       assert.exception(()=>{
           PubSub.publishSync(message, "some data");      
           clock.tick(1);
       });

       assert.exception(stub);
       assert(spy1.called);
       assert(spy2.called);
       assert(stub.calledBefore(spy1));

       clock.restore();
   });
});

Output:

Calling all the subscribers, irrespective of exceptions.

Example Of Stubs: Let us consider an example of an e-commerce website for purchasing items. If we are successful a mail will be sent to the customer.

javascript

const purchaseItems(cartItems, user)=>{
   let payStatus = user.paymentMethod(cartItems)
   if (payStatus === "successful") {
     user.SuccessMail()
   } else {
     user.redirect("error_page_of_payment")
   }
 }
}

function() {

  // Mail will be send for successful payment.
  let paymentStub = sinon.stub().returns("successful")
  let mailStub = sinon.stub(
  let user = {
    paymentMethod: paymentStub,
    SuccessMail: mailStub
  }

  purchaseItems([], user)
 
  assert(mailStub.called)
}

Example 1: A Simple Example To Execute Stubs.

html

<!DOCTYPE html>
<html>

<head>
    <title>Page Title</title>
</head>

<body>
    <div id="mocha"></div>
</body>

</html>

html

<!DOCTYPE html>
<html>

<head>
    <script>
        mocha.setup('bdd');

        function saveUser(user, callback) {
            $.post('/users', {
                first: user.firstname,
                last: user.lastname
            }, callback);
        }

        describe('saveUser', function () {
            it('should call callback after saving',
            function () {

                // We'll stub $.post so a 
                // request is not sent
                var post = sinon.stub($, 'post');
                post.yields();

                // We can use a spy as the callback 
                // so it's easy to verify
                var callback = sinon.spy();

                saveUser({ firstname: 'Han', 
                    lastname: 'Solo' }, callback);

                post.restore();
                sinon.assert.calledOnce(callback);
            });
        });

        mocha.run();
    </script>
</head>

</html>

Output:

Example 2:

html

<!DOCTYPE html>
<html>

<head>
    <title>Page Title</title>
</head>

<body>
    <h1>GeeksForGeeks</h1>
    <div id="mocha"></div>
</body>

</html>

html

<!DOCTYPE html>
<html>

<head>
    <script>
        mocha.setup('bdd');

        function saveUser(user, callback) {
            $.post('/users', {
                first: user.firstname,
                last: user.lastname
            }, callback);
        }

        describe('saveUser', function () {
            it(
'It will send the correct parameters to the expected URL',
            function () {

                // We'll stub $.post same as before
                var post = sinon.stub($, 'post');

                // We'll set up some variables to 
                // contain the expected results
                var expectedUrl = '/users';
                var expectedParams = {
                    first: 'Expected first name',
                    last: 'Expected last name'
                };

                // We can also set up the user we'll
                // save based on the expected data
                var user = {
                    firstname: expectedParams.first,
                    lastname: expectedParams.last
                }

                saveUser(user, function () { });
                post.restore();

                sinon.assert.calledWith(post, 
                    expectedUrl, expectedParams);
            });
        });

        mocha.run();
    </script>
</head>

</html>

Output:

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