Node.js

ES6

What is ES6?

• ECMAScript 6 is called ES6.

What is ECMAScript?

• It is a standard syntax of Javascript.
• Many modern syntax as been added to Javascript After ECMAScript version 6.

Why do we have to use ES6?

• Modern syntax helps us with productivity improvement.
• Node.js is supporting recent ECMAScript.
• It is important to learn useful modern syntax and utilize the syntax in an appropriate way.

Node.js and ES6 Relationship

• Not all syntax of ES6 are not supported in Node.js.

let, const

ES5

// Variable and Constant are not distinguishable in syntax
var TITLE = "NODE.js";
var director = "elice";
director = "rabbit";
TITLE = "ES6"; // no error

ES6

// We can distinguish constant and variable in syntax
const TITLE = "NODE.JS";
let director = "elice";
director = "rabbit";
TITLE = "ES6" // error

Templete String

ES5

var name = "elice";
var age = 5;
// We need "\n" to break line
// We also have to concatenate string with variable using +
var hi = `My name is `
  + name
  + '.\n I\'m'
  + age
  + 'years old.';

ES6

const name = "elice";
const age = 5;
// Easy line break.
// variable use is also easy
const hi =
`My name is ${name}.
I'm ${age} years old`;

Arrow Function

ES5

// function declaration
function doSomething(param) {
  console.log("do something");
}

// ananymous function
setTimeout(function(param) {
  console.log("no name function");
}, 0)

// We can declare same named function again
function doSomething () {
  console.log("do other");
}

ES6

const doSomething = (param) => {
  console.log("do Something");
}

setTimeout((param)=> {
  console.log("no name function");
},0);

doSomething = () => {
  console.log("do other");
}

Class

ES5

function Model(name, age){
  this.name = name;
  this.age = age;
}

Model.prototype.getInfo = function() {
  console.log(this.name, this.age); 
}
var model = new Model("elice", 5);
model.getInfo();

ES6

class Model {
  constructor (name, age){
    this.name = name;
    this.age = age;
  }

  getInfo() {
    console.log(this.name, this.age)
  }

}
const model = new  Model("elice", 5);
model.getInfo();

Destructing

ES5

var obj = {name:"elice", age:5};
var name = obj.name;
var age = obj.age;

var arr = ["some","values"];
var first = arr[0];
var second = arr[1];

ES6

const obj = {name:"elice", age:5};
const {name, age} = obj;
const {name : n1, age : a1} = obj;
const arr = ["some", "values"];
const [first, second] = arr;

Asynchronous Coding

What is Asynchronous Coding?

• It is a way to implement asynchronous even driven operations.
• There are 3 ways to implement Asynchronous operations in Node.js

What is 3 ways for Asynchronous coding in Node.js?

Callback

db.getUsers((err, users)=>{
  console.log(users);
});

// Callback hell
db.getUsers((err, users) =>{
  if(err) {...return;}
  async1(users, (r1) =>{
    async2(r1, (r2) =>{
      async3(r2, (r3) =>{
        ...
      });
    });
  });
})

• Traditional way for event driven implementation

Promise

db.getUsersPromise().then((users) =>{
  returnpromise1(users);
  })
  .then(r1 =>promise2(r1))
  .catch(...);

function getUsersPromise(params) {
  return newPromise((resolve, reject)=>{
    getUsers(params, (err, users) =>{
      if (err) {
        reject(err);return;
      }
      resolve(users)
    ;})
  ;})
;}

// promise hell can be also happened...
promise1().then(r1 => {
  return promise2(r1)
  .then(r2 =>promise3(r1, r2))
  });

• Asynchronous coding that makes up for the callback's disadvantages.
• We can omit return when There is only one line inside of a brace.
• When error occurs, promise executes .catch.
• When promise3 function needs results from 2 other functions, promise hell might also be happened.

Aync-Await

async function doSomething() =>{
  cosntr1 = awaitpromise1();
  constr2 = awaitpromise2(r1);
  constr3 = awaitpromise3(r1, r2);
  ...
  returnr3;
});

doSomething().then(r3 =>{
  console.log(r3)
});

• It is another syntax of promise
• We can get result from promise function by using await in async function.
• Until promise function using await is completed, interpreter doesn't go to next line.
• return of async function is promise function.
• Asynchronous coding that makes up for the promise's disadvantages.

Promise and Async function error handling

// promise error handling
function doSomething(msg) {
  return promise1()
    .then(r => {
      console.log(r)
    })
    .catch(e => {
      console.log(e)
    });
}


// async error handling
async function doSomething(msg){
  try {
    const r = await promise1();
    console.log(r);
  } catch(e) {
    console.error(e);
  }
}

Promise parellel execution

// This function is going to take 3 sec..
async function sync() {
  constr1 = await promise1(); // Let's say this takes 1 sec.
  constr2 = await promise2(); // Let's say this takes 2 sec.
  console.log(r1, r2);
}
---

// This function is going to take 2 sec...
async function parallel() {
  const[r1, r2] = await Promise.all([promise1(),promise2(),]); // 1 sec and 2 sec in parellel
  console.log(r1, r2);
};

In Concolusion

• We prefer async-await. It's because this syntax has a good readability
• But legacy code could be in promise or callback. So we need to be able to implement all of them in an appropriate way.