
**Introduction**

We have covered most asked interview questions and their answers, definetely this page will be help if you are preparing for the interview. Please bookmark yourself for your current or future reference.

<div class="list-number highlight">

- [What is IIFE ?](#IIFE)
- [What is hoisting ?](#hoisting)
- [What are the different ways of creating variables in Javascript ?](#let_const_var)
- [what is Arrow functions ?](#arrow)
- [How to extract unique value from an Array](#unique)
- [What is currying ?](#currying)
- [How to create object in Javascript ?](#object)
- [What are the different type of client side storages and its differences.](#client_storage)
- [What is the new way of using window object in Javascript and provide example ?](#windowObject)
- [How to check a number is Amstrong or not](#Amstrong)

</div>
<div class="list-number">

- <h3 class="text-sm" id="IIFE">What is IIFE ?</h3>

  Immediate invoke function expression is a function this will execute right after the declartion, An IIFE can be avoiding the variable hoisting within the blocks. It is also called as self executing anonymous functions.

  syntax

  ```javascript
  (function () {
    // logic goes here
  })();
  ```

- <h3 class="text-sm" id="hoisting">What is Hoisting ?</h3>

  Hoisting is moving the declaration of variable and function to the top of the current scope.

  Why hoisting is used ?

  Hoisting allows you to use the function and variable before they are declared.

  How to prevent hoising ?

  After the release of ES6, two new featurea have introduced such as <a href="/posts/what-are-es6-features#let-and-const">Let and Const.</a> If the variable or function declared any of these will prevent the hoisting, because these two has block scope and the `var` has globally scoped.

- <h3  class="text-sm" id="let_const_var">What are the different ways of creating variables in Javascript ?</h3>

  Before ES6 in Javascript <span class="highlight-small">var</span> keyword mainly used, but after the ES6 release other two way of declaration is introduced, that is Let and Const.

  The existing keyword `var` have one limitation because it has global scope and there is a chance for overwrittern the value from other places and also can use it before declaration.

  ```javascript
  var name = "abc";
  var name = 123; // will not throw any error

  country = "Mexico"; // will not throw any error about declaration because it is hoisted
  var country;
  ```

  The latest keywords let and const has block scoped, So it will not be hoisted.

  <span class="highlight-small">Let</span> - The let keyword cannot be redeclared, It will not be allow us to use it before declaration.

  ```javascript
  let name = "abc";
  let name = 123; // will throw the error, Let keyword won't allow to redeclare

  country = "Mexico"; // will throw error because cannot be able to use it before declaration
  let country;
  ```

  <span class="highlight-small">Const</span> - The const keyword cannot be redeclared and reassigned, It will not be allow us to use it before declaration.

  ```javascript
  const name = 'abc'
  name = 'cde' // will throw the error,
               // const keyword won't allow to reassign the value

  const name = 123 // will throw the error,
                   // const keyword won't allow to redeclare

  country = 'Mexico' // will throw error,
                     // because cannot be able to use it before declaration
  const country
  ```

- <h3 class="text-sm" id="arrow">What is Arrow functions ?</h3>
  Arrow function is an 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;
  ```

- <h3 class="text-sm" id="unique">How to extract unique value from an array in Javascript ?</h3>

  There are many ways to find the unique values from an array, but we have to apply more lines of code.

  Other than the longer steps we can use <span class="highlight-small">Set</span> to find the unique numbers in one line of code.

  ```javascript
  const arr = [1, 2, 3, 4, 1, 1, 2, 3];
  const uniqueArray = Array.from(new Set(arr));

  // [1, 2, 3, 4]
  ```

- <h3 class="text-sm" id="currying">What is currying ?</h3>
    Currying in JavaScript transforms a function with multiple arguments into a nested series of functions, each taking a single argument.

  ### What are the benefits of currying?

  Curried functions can make code more readable and expressive by breaking down complex logic into smaller, more manageable pieces. This can make it easier for developers to understand and maintain code. Currying allows for more flexibility in the way that functions are composed.

  ```javascript
  function calculateVolume(length) {
    return function (breadth) {
      return function (height) {
        return length * breadth * height;
      };
    };
  }

  console.log(calculateVolume(4)(5)(6));
  ```

- <h3 class="text-sm" id="object">How to create object in Javascript ?</h3>

  Javascript is not a class based object-oriented language. But it still has ways of using object oriented programming. (object oriented javascript)

  There are many ways to create objects in javascript as below

  **1.** **Object constructor:**

  The traditional way to create an empty object is using the Object constructor. But currently this approach is not recommended.

  ```javascript
  var object = new Object();
  ```

  **2.** **Object's create method:**

  The create method of Object creates a new object by passing the prototype of object as a parameter

  ```javascript
  var object = Object.create(null);
  ```

  **3.** **Object literal syntax:**

  The object literal syntax (or object initializer), is a comma-separated set of name-value pairswrapped in curly braces.

  ```javascript
  var object = {
       name: "Mr. ABC"
       age: 34
  };
  Object literal property values can be of any data type, including array, function, and nested object.
  ```

  **Note:** This is an easiest way to create an object

  **4.** **Function constructor:**
  Create any function and apply the new operator to create object instances,

  ```javascript
  function Person(name) {
    this.name = name;
    this.age = 21;
  }
  var object = new Person("Mr. ABC");
  ```

  **5.** **Function constructor with prototype:**

  This is similar to function constructor but it uses prototype for their properties and methods,

  ```javascript
  function Person() {}
  Person.prototype.name = "Mr. ABC";
  var object = new Person();
  ```

  This is equivalent to an instance created with an object create method with a functionprototype and then call that function with an instance and parameters as arguments.

  ```javascript
  function func {};
  new func(x, y, z);
  ```

  **(OR)**

  ```javascript
  // Create a new instance using function prototype.
  var newInstance = Object.create(func.prototype)
  // Call the function
  var result = func.call(newInstance, x, y, z),
  // If the result is a non-null object then use it otherwise just use the new instance.
  console.log(result && typeof result === 'object' ? result : newInstance);
  ```

  **6.** **ES6 Class syntax:**
  ES6 introduces class feature to create the objects

  ```javascript
  class Person {
    constructor(name) {
      this.name = name;
    }
  }
  var object = new Person("Mr. ABC");
  ```

  **7.** **Singleton pattern:**

  A Singleton is an object which can only be instantiated one time. Repeated calls to its constructor return the same instance and this way one can ensure that they don't accidentally create multiple instances.

  ```javascript
  var object = new (function () {
    this.name = "Mr. ABC";
  })();
  ```

- <h3 class="text-sm" id="client_storage">What are the different type of client side storages and its differences.</h3>
  There are 3 types of client side storages are available, namely Cookie, local storage and session storage.
  Each 3 type of client storages are more powerful in their feature.

    <div class="table-container">
    <div class="table-basic">
    <table>
    <th>Feature</th> <th>Cookie</th> <th>Local storage</th> <th>Session storage</th>
    <tr>
        <td>Availability</td>
        <td>Both server and client side</td>
        <td>Only at client side</td>
        <td>Only at client side</td>
    </tr>
    <tr>
        <td>Life time</td>
        <td>Available until as configured</td>
        <td>Until deleted or clear browser data</td>
        <td>Until closing the tab</td>
    </tr>
    <tr>
        <td>SSL</td>
        <td>Yes</td>
        <td>No</td>
        <td>No</td>
    </tr>
    <tr>
        <td>Maximum storage</td>
        <td>4 KB</td>
        <td>5 MB</td>
        <td>5 MB</td>
    </tr>
    </table>
    </div>
    </div>

- <h3  class="text-sm" id="windowObject">What is the new way of using window object in Javascript and provide example ?</h3>
  The globalThis property provides a standard way of accessing the global this value (and hence the global object itself) across environments. Unlike similar properties such as window and self , it's guaranteed to work in window and non-window contexts.

  ```javascript
  globalThis?.window.location.href;
  ```

- <h3 id="Amstrong" class="text-sm">How to check a number is Amstrong or not</h3>

  **What is amstrong number**</br>

  if the sum of its own digits raised to the power number of digits gives the number itself.

  ```javascript
  function amstrong(num) {
    let num1 = num;
    let sum = 0;
    while (num > 0) {
      rem = num % 10;
      sum = sum + Math.pow(rem, num1.toString().length);
      num = parseInt(num / 10);
    }
    if (sum == num1) {
      console.log(num + " is Armstrong");
    } else {
      console.log(num + " is not Armstrong");
    }
  }
  amstrong(153);
  amstrong(154);
  ```

  Result

  ```javascript
  "153 is Armstrong";
  "154 is not Armstrong";
  ```

  You might have doubt how 153 is Armstrong and why 154 is not.

  153 = (1* 1* 1)+(5 _ 5_ 5)+(3* 3* 3)  
  where:  
  (1* 1* 1)=1  
  (5* 5* 5)=125  
  (3* 3* 3)=27  
  So:  
  1+125+27=153

</div>


We covered most asked questions for Javascript interview and their answers

Javascript interview questions and answers


- ## **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

What are ES6 features ?


**Introduction**

ECMA script is commonly used for clientside on the WWW and other server side scripting like nodeJS.

There are lot of Ecma script standards which is released in various years. On each release Ecma is introduced with very good features which is so helpful to developers to reduce the development time.
 I would like to list the features of those versions.   (ES6, ES7, ES8, ES9, ES10, ES11, ES12, ES13).

## **What is ECMA script**

ECMAScript is a scripting language specification on which JavaScript is based. ECMA International is in charge of standardization of ECMAScript
<div class="highlight">

### **ES6 (ES2015)**

- let and const
- arrow functions 
- default parameters
- rest and spread operators
- template literal 
- destructoring assignment
- classes and inheritance 
- promises for asynchronous programming
- symbols for creating unique object keys 
- iteraters and generators for iterrable objects
- Sets and Maps for working with collections
<a href="/posts/what-are-es6-features" class="see-more-right">See full details</a>
</div>

<div class="highlight">

### **ES7 (ES2016)**

- Array.prototype.includes()
</div>
<div class="highlight">

### **ES8 (ES2017)**

- Object.entries()
- Object.values()
- String.prototype.padStart()
- String.prototype.padEnd()
- async/await
</div>
<div class="highlight">

### **ES9 (ES2018)**

- Object.getOwnPropertyDescriptors()
- Spread syntax for objects
- Promise.prototype.finally()
</div>
<div class="highlight">

### **ES10 (ES2019)**

- Array.prototype.flat()
- Array prototype.flatMap()
- string.prototype.trimStart()
- string.prototype.trimEnd()
- Array.prototype.sort() (stable)
</div>
<div class="highlight">

### **ES11 (ES2020)**

- BigInt()
- Nullish Coalescing operator (??)
- Optional chaining operators(?.)
- Promise.allSettled()
</div>
<div class="highlight">

### **ES12 (ES2021)**

- String.prototype.replaceAll()
- Logical assignment operators (||=,&&=,?=)
</div>
<div class="highlight">

### **ES13 (ES2022)**

- Array.prototype.lastIndexOf()
- Object.hasOwn()
- at() for string and arrays
- Top level await()
</div>

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

**Learn more**

- <a href="/posts/what-are-es6-features">What are ES6 features ?</a>
</div>

what are the new features among the various versions of ECMA script and what is difference

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