- What is Unit Testing with Mocha and Chai Study?
- What are software tests and why use them?
- What is Mocha?
- How do you install Mocha and use Mocha?
- What is Chai and how do you use in it conjunction with Mocha?
"Unit Testing with Mocha and Chai Study" is an exploration of software testing, using the Mocha framework and Chai assertion library. For the purpose of this study, I divided the study into two examples. The first "isEqual" example explores basic unit test implementation with Mocha and Chai using a simple adder function. You will find this example below in this doucment which goes step-by-step through the development process, npm package installations, etc. The second example deals with more intricate tests involving a "fizzbuzzer" function that replaces strings of text for numerical values and edge case testing.
A software test is a routine that runs the subject under test (i.e. the testing code) and determines if the code tested conforms to expectations. You want to include tests in all of your projects because it serves as documentation that illustrates the way code should be used and provides a quick way to check if any new features that have been added break your existing code (i.e. regression).
Take for example test below, which tests the "adder.js" file in this study:
$ npm test
> test-intro@1.0.0 test C:\Users\Admin\Desktop\Study-Unit-Testing-with-Mocha-and-Chai
> mocha
adder
√ should add two numbers
√ should raise error if args not numbers
2 passing (5ms)
What's interesting about the test above is that even though you dont really know how adder is implemented, you can see from the test that it should: a) add two numbers and b) raise an error is the arguments are not numbers.
"Mocha" is a testing framework that that provides functions that can be executed according to a specfic order and logs the result in the terminal window. With Mocha, you can use the command line argument "mocha" to look for and execute a test.
Mocha provides structure by using keywords like "describe" and "it", that batch into test suites and test cases. Test suites are a collection of tests all realating to a single functionality or behavior. Test cases or unit tests test the behavior of small, independent units of code, which are the smallest chuncks of meaningful, testable code that will either pass or fail.
In your project file (assuming you have already run "npm init"), install mocha to your development environment:
npm install mocha --save-dav
Additionally, you want to add the line of code in you package.json file which will allow you to run tests in Gitbash with a single command:
"scripts": {
"test":"mocha --exit"
}
This folder will hold all the test suites we want to run for our project.
test
Important conventions to remember when creating individual test files in your test folder:
- Each file should begin with "test-".
- Use dashes between each word (i.e. test-adding-code).
- The name of the file should be all lowercase.
The "subject under test" is simply the code/JavaScript file you want to test in this module. For example, suppose you want to test a function whose prupose is to do a strict comparison of two numbers. Also note that we need to export that module so that it can be used
function isEqual(a, b) {
return a === b;
}
module.exports = isEqual To load the subject under test, in your test file (e.g. test-isequal) require the file you wish to test. Also note the ".." in "../isEqual" which is the root location of the project folder.
const isEqual = require('../isEqual');To create a test suite, use describe to group all of your tests together, which take a string and a callback function. Inside the describe test suite, you can have one or more it test cases, which also has a string value and a callback function with the respective test for the behavior. Note that within the test suite, can nest as many test units as you wish.
describe('isEqual', function() {
it('should give right answers for equal and unequal inputs')
});To test this out, run npm test (remember, we added a script to our package.json file to run "mocha --exit" with this line). If you setup your test correctly, you
should see the following below. Note that the test will show "pending" because a callback function has not been supplied yet. This is where the chai assertion library comes in.
$ npm test <== when you run "npm test"...
> mocha-demo-2@1.0.0 test C:\Users\Admin\Desktop\mocha-demo <== mocha-demo (i.e. the test folder)...
> mocha --exit <== ... runs mocha --exit (i.e. the script for "test")
isEqual <== name of the function being tested...
- should give right answers for equal and unequal inputs <== first argument is a string value indicating the behavior to be tested in that block.
0 passing (9ms)
1 pending <== Test is pending because we have not supplied a callback function.
Chai is a "test expectation" library that helps you make assertions (thus you can call it an assertion library about code behavior in conjunction with Mocha. It provides functiond and methods that help compre the output of certain tests with the expected valuesyou stipulate in your tests. Chai is provides extremely clear syntax that will help you in unit testing as well.
Same process as sintalling the Mocha dependency in your package.json file, simply install the library using npm.
npm install chai --save-dev
With chai, we want to require chai so that we can utilize it in our test file.
const isEqual = require('../isEqual');
const expect = require('chai'); // load chai
describe('isEqual', function() {
it('should give right answers for equal and unequal inputs')
});In each test suite (i.e. describe), inside each unit test (i.e. it), and inside the callback function you add to it unit tests, you can use chai test the code for your unit test. In the example below, we want to test whether or not 1 and 1 are equal (rememeber, our subject to test is a function that takes 2 arguments that are compared via strict equality). Then, we simply chain together the getters to create and execute the assertion.
In the example below, we are testing "isEqual" with two test numbers, 2 and 2, which should be true. In other words, we are testing whether or not the answer generated by the subject under test conforms to expectations.
const isEqual = require('../isEqual');
const expect = require('chai');
describe('isEqual', function() {
it('should give right answers for equal and unequal inputs', function() {
expect(isEqual(2,2).to.be.true)
});
});And finally when you run your test with npm test you will see some functional results. In the case of this study, there will one of two results.
If the test conforms to expectations (i.e. 1 === 1 is true), then the test will pass. If the test does NOT conform to expectations, then the test will fail.
$ npm test
> mocha-chai@1.0.0 test C:\Users\Admin\Desktop\mocha-chai
> mocha --exit
isEqual
√ should give right answers for equal and unequal inputs
1 passing (10ms)