ES6 Interview Questions

Es6 or ECMASCRIPT 2015 is sixth major release of ECMAScript language which comes with a lot of new features and syntax for writing web applications in Javascript. As currently, not all browsers support ES6, they support pre-versions of ES6. So to write web applications in ES6 that will support all Browsers we needed tools like Babel and Webpack.

1. Support for constants (also known as “immutable variables”)
2. Block-Scope support for both variables, constants, functions
3. Arrow Functions
4. Extended Parameter Handling
5. Template Literals
6. Extended Literals
7. Enhanced Regular Expression
8. Enhanced Object Properties
9. Destructuring Assignment
10. Modules, Classes, Iterators, Generators
11. Support for Map/Set & WeakMap/WeakSet
12. Promises, Meta-Programming ,Internationalization & Localization

Read More from http://es6-features.org/

Babel is one of the most popular JavaScript transpilers and becomes the industry standard. It allows us to write ES6 code and convert it back in pre-Es6 JavaScript that browser supports.

For example look the below code snippet.
In ES6 (ECMASCRIPT 2015)

const PI = 3.141593;
PI > 3.0 ;
export{PI};

In ES5 after conversion

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
var PI = 3.141593;
PI > 3.0;
exports.PI = PI;

Installation: In order to install Babel, you require node.js and NPM. Make sure Node.js is installed on your server.

To check node installed or not run below commands on your terminal.

node -v 
npm -v

Installing Babel : We can install Babel CLI locally by running below command on terminal.

npm install --save-dev babel-cli

1. It bundles your multiple modules and packs it into a single .js file.
2. It comes with integrated dev server. A small express app for local development. You simply include one Javascript tag pointed to the server, like localhost:8080/assets/bundle.js, and get live code updating and asset management for free.

Constants also are known as immutable variables are a special type of variables whose content is not changed. In Es6 a constant is defined using const keyword. Constants in Es6 enable protection to overwrite a variable value, improve performance and helps programmers to write readable and cleaner code.

Example

In Es6

const WEBSITE_URL = "http://www.abc.com";
WEBSITE_URL="new url"; // generate an error;
console.log(WEBSITE_URL);

In prior version of Es6

//  and only in global context and not in a block scope 

Object.defineProperty(typeof global === "object" ? global : window, "WEBSITE_URL", { 
value: "http://www.abc.com", enumerable: true, 
writable:     false, 
configurable: false 
});

console.log(WEBSITE_URL);

Also, Read Best Node JS Interview Questions

Template literals are the string with embedded code and variables inside. Template literal allows concatenation and interpolation in much more comprehensive and clear in comparison with prior versions of ECMAScript.

Let see an example of concatenating a string in JavaScript.

var a="Hello";
var b="John";
var c = a+ " " + b;
Console.log(c); //outputs Hello John;

In ES6 concatenation and interpolation is done by backtick “ in a single line. To interpolate a variable simply put in to {} braces forwarded by $ sign.>/p>

// In ES6

let a="Hello";
let b="John";
let c=`${a} ${b}`;
console.log(c); //outputs Hello John;

Example :

let a =[7,8,9];
let b=[1,2,3,...a,10];
console.log(b); // [1,2,3,7,8,9,10]

So spread operator spreads the contents of variable a and concatenates it in b.

Another Example

function print(...z){
	console.log(z); 
}

print(1,2,3,4);//[1,2,3,4]

Destructing assignment in another improvement in Es6. It allows us to extract data from array and objects into separate variables.

Example

let full_name =['John','Deo'];

let [first_name,last_name]=full_name;

console.log(first_name,last_name);
// outputs John Deo

Another example

let c=[100,200,330,400];

let [a,...b]=c;

console.log(a,b);

// outputs 100 [200, 330, 400]

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

    getData() {
        console.log(this.name + " is " + this.age + " years old !");
    }
}

var user = new User("foo", 7);
s1.getData();

In modern javascript let & const are different ways of creating variables. Earlier in javascript, we use the var keyword for creating variables. let & const keyword is introduced in version ES6 with the vision of creating two different types of variables in javascript one is immutable and other is mutable.
const: It is used to create an immutable variable. Immutable variables are variables whose value is never changed in the complete life cycle of the program.
let: let is used to create a mutable variable. Mutable variables are normal variables like var that can be changed any number of time.

Set is a collection of unique values. The values could be also primitives or object references.

Creating a Set in Javascript

let set = new Set();
set.add(1);
set.add('1');
set.add({ key: 'value' });
console.log(set); // Set {1, '1', Object {key: 'value'}}

Generators are functions that can be exited and later re-entered. Their context (variable bindings) will be saved across re-entrances. A function keyword followed by an asterisk defines a generator function, which returns a Generator object.

Generator Function Example.

function* generator(i) {
  yield i;
  yield i + 10;
}

var gen = generator(10);

console.log(gen.next().value);
// expected output: 10

console.log(gen.next().value);

Further reading: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*

WeakMaps provide a way to extend objects from the outside without interfering with garbage collection. Whenever you want to extend an object but can't because it is sealed - or from an external source - a WeakMap can be applied. WeakMap was also introduced by ES6 in 2015

Further reading : WeakMap in ES6