Introduction to async/await in Node JS

1-test-mongo.js

/**
 * @author Arie M. Prasetyo
 */

const mongoose = require('mongoose');
const mongoConfig = {useNewUrlParser: true, useUnifiedTopology: true};

// connect to Mongo database
mongoose.connect('mongodb://localhost:27017/test', mongoConfig);
// define the model
const Cat = mongoose.model('Cat', {name: String});


// = = = = = = = = = = = = = = = = = = = = = = = = = = = = 

/**
 * This is a normal async javascript function
 * As you can see here, the log in line 35 was executed without waiting 
 * for the MongoDB query (line 31) to be finished.
 * This the nature of JavaScript's asynchronous behaviour.
 */
const runQuery = () => {
  Cat.find().then(value => {
    console.log(value)
  });

  console.log('run the following line\n');
}

// Run test
runQuery();

2-test-mongo-async-await.js

/**
 * @author Arie M. Prasetyo
 */

const mongoose = require('mongoose');
const mongoConfig = {useNewUrlParser: true, useUnifiedTopology: true};

// connect to Mongo database
mongoose.connect('mongodb://localhost:27017/test', mongoConfig);
// define the model
const Cat = mongoose.model('Cat', {name: String});


// = = = = = = = = = = = = = = = = = = = = = = = = = = = = 

/** 
 * If we want to do something synchronously, we can use the `async` and `await` commands.
 * Take a look at the example below:
 * 
 * This function uses async and await.
 * await is a command that will make the line to be completed before moving to the next line.
 */
const runQuery = async () => {
  const p = Cat.find().exec();
  await p.then(value => {
    console.log(value)
  });

  console.log('run the following line\n');
}

// Run test
runQuery();

3-test-sequelize.js

/**
 * @author Arie M. Prasetyo
 */

const {Sequelize, DataTypes} = require('sequelize');

// connect to MySQL database
const sequelize = new Sequelize('database_for_tests', 'root', process.env.MYSQL_PWD, {
  host: 'localhost',
  dialect: 'mysql',
  logging: false
});

// define the model
const Cat = sequelize.define('cat', {
  name: {
    type: DataTypes.STRING
  }
}, {
  timestamps: false,
  freezeTableName: true,
  tableName: 'cat'
});


// = = = = = = = = = = = = = = = = = = = = = = = = = = = = 

/**
 * Run the query in a default asynchronous manner
 */
const runQuery = () => {
  Cat
  .findAll({raw: true})
  .then(value => {
    console.log(value);
  });

  console.log('run the following line\n'); // This line will be executed before the query in line 30 is completed
}

// Run test
runQuery();

4-test-sequelize-async-await.js

/**
 * @author Arie M. Prasetyo
 */

const {Sequelize, DataTypes} = require('sequelize');

// connect to MySQL database
const sequelize = new Sequelize('database_for_tests', 'root', process.env.MYSQL_PWD, {
  host: 'localhost',
  dialect: 'mysql',
  logging: false
});

// define the model
const Cat = sequelize.define('cat', {
  name: {
    type: DataTypes.STRING
  }
}, {
  timestamps: false,
  freezeTableName: true,
  tableName: 'cat'
});


// = = = = = = = = = = = = = = = = = = = = = = = = = = = = 

/**
 * Run the query in a "synchronous" manner using async await.
 * There are several variations of how we can access the data
 * returned by the database query
 */
const runQuery = async () => {
  // Variation A
  /*
  let p = Cat.findAll({raw: true}) // get only the data
  await p.then(value => {
    console.log(value);
  });
  */

  // Variation B
  /*
  await Cat
  .findAll({raw: true}) // get only the data
  .then(value => {
    console.log(value);
  });
  */

  // Variation C
  let cat = await Cat
  .findAll({raw: true}) // get only the data
  .then(value => {
    return value;
  });
  
  console.log(cat);

  console.log('run the following line\n');
}

// Run test
runQuery();