Mocha is a test framework to run tests.

https://mochajs.org/#assertions

Chai is an assertion library and you can find full documentation in the following link.

https://www.chaijs.com/api/bdd/

There are three type of assertion libraries in Chai.

• Expect
• Should
• Assert

expect and should use the same chainable language to construct assertions.

Assume that expect library is used like below

expect(firstName).to.be.a('string');
expect(firstName).to.equal('kenan');
expect(firstName).to.have.lengthOf(5);
expect(beverages).to.have.property('tea').with.lengthOf(3);

Above code could be written with should library like below.

firstName.should.be.a('string');
firstName.should.equal('kenan');
firstName.should.have.lengthOf(5);
beverages.should.have.property('tea').with.lengthOf(3);

You can find more detailed usage with Express, Mocha and Chai in the following repository

https://github.com/nodejs-projects-kenanhancer/express-demo.git

You can find all the usages in the following code.

const chai = require('chai');
const { expect, assert } = chai;
const should = chai.should();

const firstName = 'kenan';
const beverages = { tea: [ 'chai', 'matcha', 'oolong' ] };



describe('Chai Expect Usage',()=>{
  it('Expect Simple Usage',()=>{
    expect(firstName).to.be.a('string');
    expect(firstName).to.equal('kenan');
    expect(firstName).to.have.lengthOf(5);
    expect(beverages).to.have.property('tea').with.lengthOf(3);
  });
});

describe('Chai Should Usage', ()=>{
  it('Should Simple Usage', ()=>{
    firstName.should.be.a('string');
    firstName.should.equal('kenan');
    firstName.should.have.lengthOf(5);
    beverages.should.have.property('tea').with.lengthOf(3);
  });
});

describe('Chai Assert Usage', ()=>{
  it('Assert Simple Usage',()=>{
    assert.typeOf(firstName, 'string');
    assert.typeOf(firstName, 'string','firstName is a string');
    assert.equal(firstName, 'kenan', 'firstName equal `kenan`');
    assert.lengthOf(firstName, 5, 'firstName`s value has a length of 5');
    assert.lengthOf(beverages.tea, 3, 'beverages has 3 types of tea');
  });
});

describe('pow', function(){
  it('raises to n-th power', function(){
    assert.equal(Math.pow(2,3), 8);
    expect(Math.pow(2,3)).to.equal(8);
    Math.pow(2,3).should.equal(8);
  });
});

describe('Array', ()=>{
  describe('#indexOf', ()=>{
    it('it should return -1 when the value is not present', ()=>{
      const myArray = [1,2,3];
      
      assert.isArray(myArray, 'is array of numbers');
      assert.include(myArray, 2, 'array contains 2');
      assert.lengthOf(myArray, 3, 'array contains 3 numbers');
      assert.equal(myArray.indexOf(4), -1);

      expect(myArray).to.be.an('array').that.includes(2);
      expect(myArray).to.have.lengthOf(3);
      expect(myArray.length).to.equal(3);

      myArray.should.be.an('array').that.includes(2);
      myArray.should.have.lengthOf(3);
      myArray.length.should.equal(3);
    });
  });
});

Same code can be executed in the following repl editor.To see unit tests open myTest.js file.

More examples are in the following demo. To see unit tests open myTest.js file.