
- ## **Introduction**
  ES6 or the ECMAScript 2015 is the major edition of ECMAScript language, which is introduced the year 2015. It introduced several new features, that can be useful for developers to fasten the development.

There are more than 10 feature introduced with the release 2015. I have listed the below the features and the details.

<div class="highlight">

<h3 class="font-bold" id="ES6">ES6 (ES2015)</h3>
<div class="list-square">

- [Let and const](#let-and-const)
- [Arrow functions](#Arrow-functions)
- [Default parameters](#Default)
- [Rest and spread operators](#rest-spread)
- [Template literal](#template-literal)
- [Destructoring assignment](#destructoring-assignment)
- [Classes and inheritance](#classes)
- [Promises for asynchronous programming](#promises)
- [Symbols for creating unique object keys](#symbols)
- [Iteraters and generators for iterrable objects](#iteraters-generators)
- [Sets and Maps for working with collections](#Sets-Maps)
</div>
</div>

<h3 class="font-bold" id="let-and-const">Let and const</h3>

The keywords <span class="highlight-small">`let and const`</span> are used to define variables and constants. Before ES6, only the `var` keyword was used to declare the variable and this has function scope. `var` will be hoisted.hoisted means the variable declared using `var` will move to the top and this variable will be available to use before declaring.

The `let` variables and `const` have block scope which is surrounded by curly-braces "{}" and cannot be used before declaration.

E.g:-

```javascript
sampleVariable = 10; // this variable used before declaring, but it is not going to throw the error
// this is restricted in let and const feature
var sampleVariable;
```

let and const example

E.g:-

```javascript
sampleVariable = 10; // this will throw, because the operation is not allowed before decalration
let sampleVariable;
```

The correct usage is given below.

```javascript
let sampleVariable;
sampleVariable = 10;
console.log(sampleVariable); //output 10
```

const - const variable must be assigned the value when declaring the variable itself and this cannot be changed.

```javascript
const sampleVariable;       // empty declaration is not allowed
const sampleVariable=10     // correct
console.log(sampleVariable) // output 10
```

[⬆ Back to Top](#ES6)

<h3 class="font-bold" id="Arrow-functions">Arrow functions</h3>

The arrow function is another feature of ES6. This is used to write the function shorter. Arrow functions are defined using the fat arrow (=>) notation.

```javascript
const add = (number1, number2) => number1 + number2;
```

[⬆ Back to Top](#ES6)

<h3 class="font-bold" id="Default">Default Parameters</h3>

A function is declared with some parameters and when the function is used somewhere in the program without providing the expected params, then the compiler will throw out the error with 'undefined'. So, to prevent this situation, ES6 is allowing the programmer to set the default value. This value will take place when the programmer missed to provide the parameter.

E.g:-

```javascript
const add = (number1, number2) => number1 + number2;
console.log(add()); // Error, input parameters are not provided

const add = (number1 = 10, number2 = 10) => number1 + number2;
console.log(add()); // result will be 20

const add = (number1 = 10, number2 = 10) => number1 + number2;
console.log(add(15, 15)); // result will be 30
```

[⬆ Back to Top](#ES6)

<h3 class="font-bold" id="rest-spread">Rest and Spread operators</h3>

The <span class="highlight-small">`rest`</span> operator allow us to accept indefinite number of parameter as an array to a function.

```javascript
function(param1, ...args)
{
   for(let i in args)
   {
     console.log(i)
   }

}
```

The <span class="highlight-small">`spread`</span> operators used to take deep copy of JS objects.

```javascript
let originalValues = ["a", "b", "c"];
let copyValues = [...originalValues];
console.log(copyValues);
```

[⬆ Back to Top](#ES6)

<h3 class="font-bold" id="template-literal">Template literals</h3>

The template Literals use back-ticks (``) rather than the quotes ("") to define a string

```javascript
let title = `template literals is using back-ticks`;
```

It is also allowing multiline declaration, so no more <span class="highlight-small">`+`</span> symbol to append the lines

```javascript
let title = `template 
             literals is 
             using back-ticks`;
```

It is also allowing the interpolation on strings.

```javascript
let topic = `Template literals`;
let ECMAVersion = `ES6`;
let statement = `The ${topic} is a feature of ${ECMAVersion}`;

console.log(statement);
// The Template literals is a feature of ES6
```

[⬆ Back to Top](#ES6)

<h3 class="font-bold" id="destructoring-assignment">Destructoring assignment</h3>

This is one of the good features of ES6. It enables us to extract the relevant details from an array or object.

```javascript
// Array destructoring
let RGB = ["RED", "BLUE", "GREEN"];
let [R, G, B] = RGB;
console.log(R); // RED
console.log(G); // GREEN
console.log(B); // BLUE
```

```javascript
// Object destructoring
const UserDetails = {
  name: { firstName: "John", lastName: "Jon" },
  phone: "8982928292",
  email: "johnjon@abc.com",
  country: "India",
};

const [name, email] = UserDetails;
```

[⬆ Back to Top](#ES6)

<h3 class="font-bold" id="classes">Classes and inheritance</h3>

The class was not used in earlier JavaScript, this class. This looks the same as other object-oriented languages. To inherit the class, for others, the <span class="highlight-small">`extend`</span> keyword is used.

```javascript
class EcmaScript
{
    constuctor(feature, version)
    {
        this.feature = feature
        this.version = version
    }
    const notes = () => console.log(the ${feature} is introduced in the version of ${version})

}

let obj = new EcmaScript('Class','ES6')
obj.notes()

// the Class is introduced in the version of ES6
```

[⬆ Back to Top](#ES6)

<h3 class="font-bold" id="promises">Promises for asynchronous programming</h3>

A promise function enables an asynchronous operation which will produce a result later either rejected or resolved.There are 3 states for a <span class="highlight-small">`promise`</span> operation. Those are pending, fulfilled and rejected.

**Pending -** The result of the operation is not computed, so this pending will be the initial state of a Promise.

**Fulfilled -** The operation is completed as expected.

**Rejected -** The operation did not go well as expected.

Syntax

```javascript
const Promise = new Promise((resolve,reject) => {....});
```

E.g:-

```javascript
let Promise = new Promise((resolve, reject) => {
  let result = fetch(url);
  if (result.data !== undefine) {
    resolve("Success");
  } else {
    reject("Failed");
  }
});
Promise.then((message) => {
  console.log("The data is restrived successfully");
}).catch((message) => {
  console.log("error");
});
```

[⬆ Back to Top](#ES6)

<h3 class="font-bold" id="symbols">Symbols for creating unique object keys</h3>

Symbols, newly added to ES6, are a primitive data-type that gives a unique value (Symbol value) as an output.

E.g:-

```javascript
let id = Symbol("id");

let emp={
    name:'abc'
    [id]:111
}
console.log(emp[id])
```

[⬆ Back to Top](#ES6)

<h3 class="font-bold" id="iteraters-generators">Iteraters and generators for iterrable objects</h3>

The iterators like the loop that we used earlier in the array. The same can be used in strings and objects with the help of <span class="highlight-small">`for..of`</span>

```javascript
const name = "devphobia";

for (const x of name) {
  console.log(x);
}

Result;
d;
e;
v;
p;
h;
o;
b;
i;
a;
```

The ES6 generator is a different kind of function that may be paused in the middle either one or many times and can be resumed later. When the standard function is called, the control rests with the called function until it returns, but the generator in ES6 allows the caller function to control the execution of a called function.

[⬆ Back to Top](#ES6)

<h3 class="font-bold" id="Sets-Maps">Sets and Maps for working with collections</h3>

A <span class="highlight-small">`Set`</span> is a data structure that allows you to create a collection of unique values. `Set` is the collections that deal with single objects or single values. It supports both primitive values and object references.

```javascript
let colors = new Set(['Green', 'Red', 'Orange', 'Yellow', 'Red']);
console.log(colors.size);
console.log(colors);

Result
4
Set { 'Green', 'Red', 'Orange', 'Yellow' }
let map = new Map([iterable]);
```

The <span class="highlight-small">`Map`</span> is a new collection type for JavaScript. It will hold any type of value like a key value pair. The map remembers the insertion order of keys.

```javascript
let map = new Map([iterable]);
```

<div class="learn-more-container">

**Learn more**

- <a href="/posts/Different-versions-of-ecma-script">Different versions of the ECMAscript</a>
</div>


ES6 or the ECMAScript 2015 is the major edition of ECMAScript language, it introduced several new features which are very special to the developers

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