test.js

const assert = require('assert');

function increment(value) {
  return new Promise((resolve, reject) => {
    if (value < 0) {
      reject(Error('I can only increment positive numbers'));
    } else {
      resolve(value + 1);
    }
  });
}

describe('increment', () => {

  it('should increment the value by 1', () => {
    return increment(123).then(value => {
      assert.equal(value, 124);
    });
  });

  it('should fail when incrementing negative numbers', () => {
    return increment(-123).then(value => {
      throw Error('it should have failed');
    }, err => {
      // swallow the error
    });
  });

  // this test will intentionally fail
  it('should not increment the value by 2', () => {
    return increment(123).then(value => {
      assert.equal(value, 125);
    });
  });

});