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Mocha & Chai

Basic setup​

Installation​

npm i --save-dev mocha chai

Create test file​

  • if the file you wanna test is named as abc.js, create the same file but ends with test.js
  • save all test files into a test folder

    abc.test.js

Enable ES6 support and to ensure it is transpiled correctly by installing babel​

  • not required if not using import and export from ES6

    npm i --save-dev @babel/core @babel/preset-env @babel/register

Create a .babelrc file​

  • the content of the file must have the following
{
  "presets": ["@babel/preset-env"]
}

run tests​

  • without ES6 support

    mocha "test/*/.js"

  • with ES6

    npx mocha "test/*/.test.js" --recursive --require @babel/register

save the above command in the package.json script test area​

"scripts": {
  "test": "npx mocha 'test/**/*.test.js' --recursive --require @babel/register"
},
  • run test with npm test

run test automatically​

  • add -watch into the script in the package.json file
"scripts": {
  "test": "npx mocha 'test/**/*.test.js' --recursive --require @babel/register -watch"
},

Mocha Keywords​

describe​

  • Use to group similar tests together

  • it is a function that takes 2 basic arguments

    • 1st arg is a string used to describe the group of tests we are running

      • example:

        import {getLetterCount} from "./letter-count";

        describe("getLetterCount - basic functionality");

    • 2nd arg is a function that contains the actual tests we wanna run

      • inside this function, we can write more describes to curther categorize our tests

      • or write a mocha function called "it"

        • example:

          import {getLetterCount} from "./letter-count";

          describe("getLetterCount - basic functionality", () => {
            it();
          });

it​

  • use to denote individual tests / individual pieces of functionality that we want to test

  • it is a function that takes 2 basic arguments

    • 1st arg is a string to tell us what exact piece of functionality this test will be covering

      • example:

        import {getLetterCount} from "./letter-count";

        describe("getLetterCount - basic functionality", () => {
          it("returns an empty object when passed an empty string");
        });

    • 2nd arg is a function that contains the actual claims (assertions) we want to make about our code

      • Need to use the "expect" keyword from chai library to make the assertions

        • example:

          import {expect} from "chai";
          import {getLetterCount} from "./letter-count";

          describe("getLetterCount - basic functionality", () => {
            it("returns an empty object when passed an empty string", () => {
              const expected = {};
              const actual = getLetterCount("");
              expect(actual).to.deep.equal(expected);
            });
          });

Chai keywords​

expect​

  • to.deep.equal
    • compare 2 different objects
      • set actual to store the actual value of the return value of the function we want to test
      • set expected as the value we expect the actual variable to be
      • must use "deep" because we are comparing objects, which are stored at different memory locations
        • if "deep" is not used, result will be false even if the values are equal
          • result will only return true if both objects are pointed to the same memory location
        expect(actual).to.deep.equal(expected);
  • to.equal
    • compare 2 values (numbers, string, boolean, etc.)
    expect(actual).to.equal(expected);
  • to.be.a
    • check type ("function", "string", "object", "array", "null", "undefined", "error", "promise", etc.)
    expect(actual).to.be.a("function"); // words not starting with a vowel uses "a"
    expect([]).to.be.an("array"); // object, array, undefined, error uses "an"