JavaScript Interview Questions & Answers

Table of Contents

1. What are the possible ways to create objects in JavaScript

   There are many ways to create objects in javascript as below

   1. Object constructor:

      The simplest way to create an empty object is using the Object constructor. Currently this approach is not recommended.

      var object = new Object();
      

   2. Object's create method:

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

      var object = Object.create(null);
      

   3. Object literal syntax:

      The object literal syntax is equivalent to create method when it passes null as parameter

      var object = {};
      

   4. Function constructor:

      Create any function and apply the new operator to create object instances,

      function Person(name){
         var object = {};
         object.name=name;
         object.age=21;
         return object;
      }
      var object = new Person("Sudheer");
      

   5. Function constructor with prototype:

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

      function Person(){}
      Person.prototype.name = "Sudheer";
      var object = new Person();
      

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

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

      (OR)

      // 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

      class Person {
         constructor(name) {
            this.name = name;
         }
      }
      
      var object = new Person("Sudheer");
      

   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.

      var object = new function(){
         this.name = "Sudheer";
      }
      

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2. What is a prototype chain

   Prototype chaining is used to build new types of objects based on existing ones. It is similar to inheritance in a class based language. The prototype on object instance is available through Object.getPrototypeOf(object) or proto property whereas prototype on constructors function is available through object.prototype.

   

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3. What is the difference between Call, Apply and Bind

   The difference between Call, Apply and Bind can be explained with below examples,

   Call: The call() method invokes a function with a given this value and arguments provided one by one

    var employee1 = {firstName: 'John', lastName: 'Rodson'};
    var employee2 = {firstName: 'Jimmy', lastName: 'Baily'};
   
    function invite(greeting1, greeting2) {
        console.log(greeting1 + ' ' + this.firstName + ' ' + this.lastName+ ', '+ greeting2);
    }
   
    invite.call(employee1, 'Hello', 'How are you?'); // Hello John Rodson, How are you?
    invite.call(employee2, 'Hello', 'How are you?'); // Hello Jimmy Baily, How are you?
   

   Apply: Invokes the function and allows you to pass in arguments as an array

    var employee1 = {firstName: 'John', lastName: 'Rodson'};
    var employee2 = {firstName: 'Jimmy', lastName: 'Baily'};
   
    function invite(greeting1, greeting2) {
        console.log(greeting1 + ' ' + this.firstName + ' ' + this.lastName+ ', '+ greeting2);
    }
   
    invite.apply(employee1, ['Hello', 'How are you?']); // Hello John Rodson, How are you?
    invite.apply(employee2, ['Hello', 'How are you?']); // Hello Jimmy Baily, How are you?
   

   bind: returns a new function, allowing you to pass in an array and any number of arguments

    var employee1 = {firstName: 'John', lastName: 'Rodson'};
    var employee2 = {firstName: 'Jimmy', lastName: 'Baily'};
   
    function invite(greeting1, greeting2) {
        console.log(greeting1 + ' ' + this.firstName + ' ' + this.lastName+ ', '+ greeting2);
    }
   
    var inviteEmployee1 = invite.bind(employee1);
    var inviteEmployee2 = invite.bind(employee2);
    inviteEmployee1('Hello', 'How are you?'); // Hello John Rodson, How are you?
    inviteEmployee2('Hello', 'How are you?'); // Hello Jimmy Baily, How are you?
   

   Call and apply are pretty interchangeable. Both execute the current function immediately. You need to decide whether it’s easier to send in an array or a comma separated list of arguments. You can remember by treating Call is for comma (separated list) and Apply is for Array. Whereas Bind creates a new function that will have this set to the first parameter passed to bind().

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4. What is JSON and its common operations

   JSON is a text-based data format following JavaScript object syntax, which was popularized by Douglas Crockford. It is useful when you want to transmit data across a network and it is basically just a text file with an extension of .json, and a MIME type of application/json Parsing: Converting a string to a native object

    JSON.parse(text)
   

   Stringification: **converting a native object to a string so it can be transmitted across the network

    JSON.stringify(object)
   

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5. What is the purpose of the array slice method

   The slice() method returns the selected elements in an array as a new array object. It selects the elements starting at the given start argument, and ends at the given optional end argument without including the last element. If you omit the second argument then it selects till the end. Some of the examples of this method are,

    let arrayIntegers = [1, 2, 3, 4, 5];
    let arrayIntegers1 = arrayIntegers.slice(0,2); // returns [1,2]
    let arrayIntegers2 = arrayIntegers.slice(2,3); // returns [3]
    let arrayIntegers3 = arrayIntegers.slice(4); //returns [5]
   

   Note: Slice method won't mutate the original array but it returns the subset as a new array.

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6. What is the purpose of the array splice method

   The splice() method is used either adds/removes items to/from an array, and then returns the removed item. The first argument specifies the array position for insertion or deletion whereas the option second argument indicates the number of elements to be deleted. Each additional argument is added to the array. Some of the examples of this method are,

    let arrayIntegersOriginal1 = [1, 2, 3, 4, 5];
    let arrayIntegersOriginal2 = [1, 2, 3, 4, 5];
    let arrayIntegersOriginal3 = [1, 2, 3, 4, 5];
   
    let arrayIntegers1 = arrayIntegersOriginal1.splice(0,2); // returns [1, 2]; original array: [3, 4, 5]
    let arrayIntegers2 = arrayIntegersOriginal2.splice(3); // returns [4, 5]; original array: [1, 2, 3]
    let arrayIntegers3 = arrayIntegersOriginal3.splice(3, 1, "a", "b", "c"); //returns [4]; original array: [1, 2, 3, "a", "b", "c", 5]
   

   Note: Splice method modifies the original array and returns the deleted array.

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7. What is the difference between slice and splice

   Some of the major difference in a tabular form

   | Slice | Splice | |---- | --------- | Doesn't modify the original array(immutable) | Modifies the original array(mutable) | | Returns the subset of original array | Returns the deleted elements as array | | Used to pick the elements from array | Used to insert or delete elements to/from array|

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8. How do you compare Object and Map

   Objects are similar to Maps in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Due to this reason, Objects have been used as Maps historically. But there are important differences that make using a Map preferable in certain cases.

   1. The keys of an Object are Strings and Symbols, whereas they can be any value for a Map, including functions, objects, and any primitive.
   2. The keys in Map are ordered while keys added to Object are not. Thus, when iterating over it, a Map object returns keys in order of insertion.
   3. You can get the size of a Map easily with the size property, while the number of properties in an Object must be determined manually.
   4. A Map is an iterable and can thus be directly iterated, whereas iterating over an Object requires obtaining its keys in some fashion and iterating over them.
   5. An Object has a prototype, so there are default keys in the map that could collide with your keys if you're not careful. As of ES5 this can be bypassed by using map = Object.create(null), but this is seldom done.

   6. A Map may perform better in scenarios involving frequent addition and removal of key pairs.

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9. What is the difference between == and === operators

   JavaScript provides both strict(===, !==) and type-converting(==, !=) equality comparison. The strict operators take type of variable in consideration, while non-strict operators make type correction/conversion based upon values of variables. The strict operators follow the below conditions for different types,

   1. Two strings are strictly equal when they have the same sequence of characters, same length, and same characters in corresponding positions.
   2. Two numbers are strictly equal when they are numerically equal. i.e, Having the same number value. There are two special cases in this,
      1. NaN is not equal to anything, including NaN.
      2. Positive and negative zeros are equal to one another.
   3. Two Boolean operands are strictly equal if both are true or both are false.
   4. Two objects are strictly equal if they refer to the same Object.

   5. Null and Undefined types are not equal with ===, but equal with ==. i.e, null===undefined --> false but null==undefined --> true

      Some of the example which covers the above cases,

      0 == false   // true
      0 === false  // false
      1 == "1"     // true
      1 === "1"    // false
      null == undefined // true
      null === undefined // false
      '0' == false // true
      '0' === false // false
      []==[] or []===[] //false, refer different objects in memory
      {}=={} or {}==={} //false, refer different objects in memory
      

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10. What are lambda or arrow functions

    An arrow function is a shorter syntax for a function expression and does not have its own this, arguments, super, or new.target. These functions are best suited for non-method functions, and they cannot be used as constructors.

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11. What is a first class function

    In Javascript, functions are first class objects. First-class functions means when functions in that language are treated like any other variable.

    For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable. For example, in the below example, handler functions assigned to a listener

    const handler = () => console.log ('This is a click handler function');
    document.addEventListener ('click', handler);
    

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12. What is a first order function

    First-order function is a function that doesn’t accept another function as an argument and doesn’t return a function as its return value.

    const firstOrder = () => console.log ('I am a first order function!');
    

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13. What is a higher order function

    Higher-order function is a function that accepts another function as an argument or returns a function as a return value.

    const firstOrderFunc = () => console.log ('Hello I am a First order function');
    const higherOrder = ReturnFirstOrderFunc => ReturnFirstOrderFunc ();
    higherOrder (firstOrderFunc);
    

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14. What is a unary function

    Unary function (i.e. monadic) is a function that accepts exactly one argument. Let us take an example of unary function. It stands for a single argument accepted by a function.

    const unaryFunction = a => console.log (a + 10); // Add 10 to the given argument and display the value
    

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15. What is the currying function

    Currying is the process of taking a function with multiple arguments and turning it into a sequence of functions each with only a single argument. Currying is named after a mathematician Haskell Curry. By applying currying, a n-ary function turns it into a unary function. Let's take an example of n-ary function and how it turns into a currying function

    const multiArgFunction = (a, b, c) => a + b + c;
    const curryUnaryFunction = a => b => c => a + b + c;
    curryUnaryFunction (1); // returns a function: b => c =>  1 + b + c
    curryUnaryFunction (1) (2); // returns a function: c => 3 + c
    curryUnaryFunction (1) (2) (3); // returns the number 6
    

    Curried functions are great to improve code reusability and functional composition.

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16. What is a pure function

    A Pure function is a function where the return value is only determined by its arguments without any side effects. i.e, If you call a function with the same arguments 'n' number of times and 'n' number of places in the application then it will always return the same value. Let's take an example to see the difference between pure and impure functions,

    //Impure
    let numberArray = [];
    const impureAddNumber = number => numberArray.push (number);
    //Pure
    const pureAddNumber = number => argNumberArray =>
      argNumberArray.concat ([number]);
    
    //Display the results
    console.log (impureAddNumber (6)); // returns 1
    console.log (numberArray); // returns [6]
    console.log (pureAddNumber (7) (numberArray)); // returns [6, 7]
    console.log (numberArray); // returns [6]
    

    As per above code snippets, Push function is impure itself by altering the array and returning an push number index which is independent of parameter value. Whereas Concat on the other hand takes the array and concatenates it with the other array producing a whole new array without side effects. Also, the return value is a concatenation of the previous array. Remember that Pure functions are important as they simplify unit testing without any side effects and no need for dependency injection. They also avoid tight coupling and make it harder to break your application by not having any side effects. These principles are coming together with Immutability concept of ES6 by giving preference to const over let usage.

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17. What is the purpose of the let keyword

    The let statement declares a block scope local variable. Hence the variables defined with let keyword are limited in scope to the block, statement, or expression on which it is used. Whereas variables declared with the var keyword used to define a variable globally, or locally to an entire function regardless of block scope. Let's take an example to demonstrate the usage,

    let counter = 30;
    if (counter === 30) {
      let counter = 31;
      console.log(counter); // 31
    }
    console.log(counter); // 30 (because if block variable won't exist here)
    

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18. What is the difference between let and var

    You can list out the differences in a tabular format

    | var | let | |---- | --------- | It is been available from the beginning of JavaScript | Introduced as part of ES6 | | It has function scope | It has block scope | | Variables will be hoisted | Hoisted but not initialized |

    Let's take an example to see the difference,

    function userDetails(username) {
       if(username) {
         console.log(salary); // undefined(due to hoisting)
         console.log(age); // error: age is not defined
         let age = 30;
         var salary = 10000;
       }
       console.log(salary); //10000 (accessible to due function scope)
       console.log(age); //error: age is not defined(due to block scope)
    }
    

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19. What is the reason to choose the name let as a keyword

    Let is a mathematical statement that was adopted by early programming languages like Scheme and Basic. It has been borrowed from dozens of other languages that use let already as a traditional keyword as close to var as possible.

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20. How do you redeclare variables in switch block without an error

    If you try to redeclare variables in a switch block then it will cause errors because there is only one block. For example, the below code block throws a syntax error as below,

    let counter = 1;
    switch(x) {
      case 0:
        let name;
        break;
    
      case 1:
        let name; // SyntaxError for redeclaration.
        break;
    }
    

    To avoid this error, you can create a nested block inside a case clause and create a new block scoped lexical environment.

    let counter = 1;
        switch(x) {
          case 0: {
            let name;
            break;
          }
          case 1: {
            let name; // No SyntaxError for redeclaration.
            break;
          }
        }
    

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21. What is the Temporal Dead Zone

    The Temporal Dead Zone is a behavior in JavaScript that occurs when declaring a variable with the let and const keywords, but not with var. In ECMAScript 6, accessing a let or const variable before its declaration (within its scope) causes a ReferenceError. The time span when that happens, between the creation of a variable’s binding and its declaration, is called the temporal dead zone. Let's see this behavior with an example,

    function somemethod() {
      console.log(counter1); // undefined
      console.log(counter2); // ReferenceError
      var counter1 = 1;
      let counter2 = 2;
    }
    

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22. What is IIFE(Immediately Invoked Function Expression)

    IIFE (Immediately Invoked Function Expression) is a JavaScript function that runs as soon as it is defined. The signature of it would be as below,

    (function ()
        {
          // logic here
        }
     )
    ();
    

    The primary reason to use an IIFE is to obtain data privacy because any variables declared within the IIFE cannot be accessed by the outside world. i.e, If you try to access variables with IIFE then it throws an error as below,

    (function ()
            {
              var message = "IIFE";
              console.log(message);
            }
     )
    ();
    console.log(message); //Error: message is not defined
    

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23. What is the benefit of using modules

    There are a lot of benefits to using modules in favour of a sprawling. Some of the benefits are,

    1. Maintainability
    2. Reusability
    3. Namespacing

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24. What is memoization

    Memoization is a programming technique which attempts to increase a function’s performance by caching its previously computed results. Each time a memoized function is called, its parameters are used to index the cache. If the data is present, then it can be returned, without executing the entire function. Otherwise the function is executed and then the result is added to the cache. Let's take an example of adding function with memoization,

    const memoizAddition = () => {
      let cache = {};
     return (value) => {
      if (value in cache) {
       console.log('Fetching from cache');
       return cache[value]; // Here, cache.value cannot be used as property name starts with the number which is not a valid JavaScript  identifier. Hence, can only be accessed using the square bracket notation.
      }
      else {
       console.log('Calculating result');
       let result = value + 20;
       cache[value] = result;
       return result;
      }
     }
    }
    // returned function from memoizAddition
    const addition = memoizAddition();
    console.log(addition(20)); //output: 40 calculated
    console.log(addition(20)); //output: 40 cached
    

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25. What is Hoisting

    Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution. Remember that JavaScript only hoists declarations, not initialisation. Let's take a simple example of variable hoisting,

    console.log(message); //output : undefined
    var message = 'The variable Has been hoisted';
    

    The above code looks like as below to the interpreter,

    var message;
    console.log(message);
    message = 'The variable Has been hoisted';
    

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26. What are classes in ES6

    In ES6, Javascript classes are primarily syntactic sugar over JavaScript’s existing prototype-based inheritance. For example, the prototype based inheritance written in function expression as below,

    function Bike(model,color) {
        this.model = model;
        this.color = color;
    }
    
    Bike.prototype.getDetails = function() {
        return this.model + ' bike has' + this.color + ' color';
    };
    

    Whereas ES6 classes can be defined as an alternative

    class Bike{
      constructor(color, model) {
        this.color= color;
        this.model= model;
      }
    
      getDetails() {
        return this.model + ' bike has' + this.color + ' color';
      }
    }
    

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27. What are closures

    A closure is the combination of a function and the lexical environment within which that function was declared. i.e, It is an inner function that has access to the outer or enclosing function’s variables. The closure has three scope chains

    1. Own scope where variables defined between its curly brackets
    2. Outer function’s variables
    3. Global variables Let's take an example of closure concept,

    function Welcome(name){
      var greetingInfo = function(message){
       console.log(message+' '+name);
      }
    return greetingInfo();
    }
    var myFunction = Welcome('John');
    myFunction('Welcome '); //Output: Welcome John
    myFunction('Hello Mr.'); //output: Hello Mr.John
    

    As per the above code, the inner function(greetingInfo) has access to the variables in the outer function scope(Welcome) even after the outer function has returned.

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28. What are modules

    Modules refer to small units of independent, reusable code and also act as the foundation of many JavaScript design patterns. Most of the JavaScript modules export an object literal, a function, or a constructor

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29. Why do you need modules

    Below are the list of benefits using modules in javascript ecosystem

    1. Maintainability
    2. Reusability
    3. Namespacing

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30. What is scope in javascript

    Scope is the accessibility of variables, functions, and objects in some particular part of your code during runtime. In other words, scope determines the visibility of variables and other resources in areas of your code.

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31. What is a service worker

    A Service worker is basically a script (JavaScript file) that runs in the background, separate from a web page and provides features that don't need a web page or user interaction. Some of the major features of service workers are Rich offline experiences(offline first web application development), periodic background syncs, push notifications, intercept and handle network requests and programmatically managing a cache of responses.

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32. How do you manipulate DOM using a service worker

    Service worker can't access the DOM directly. But it can communicate with the pages it controls by responding to messages sent via the postMessage interface, and those pages can manipulate the DOM.

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33. How do you reuse information across service worker restarts

    The problem with service worker is that it gets terminated when not in use, and restarted when it's next needed, so you cannot rely on global state within a service worker's onfetch and onmessage handlers. In this case, service workers will have access to IndexedDB API in order to persist and reuse across restarts.

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34. What is IndexedDB

    IndexedDB is a low-level API for client-side storage of larger amounts of structured data, including files/blobs. This API uses indexes to enable high-performance searches of this data.

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35. What is web storage

    Web storage is an API that provides a mechanism by which browsers can store key/value pairs locally within the user's browser, in a much more intuitive fashion than using cookies. The web storage provides two mechanisms for storing data on the client.

    1. Local storage: It stores data for current origin with no expiration date.
    2. Session storage: It stores data for one session and the data is lost when the browser tab is closed.

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36. What is a post message

    Post message is a method that enables cross-origin communication between Window objects.(i.e, between a page and a pop-up that it spawned, or between a page and an iframe embedded within it). Generally, scripts on different pages are allowed to access each other if and only if the pages follow same-origin policy(i.e, pages share the same protocol, port number, and host).

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37. What is a Cookie

    A cookie is a piece of data that is stored on your computer to be accessed by your browser. Cookies are saved as key/value pairs. For example, you can create a cookie named username as below,

    document.cookie = "username=John";
    

    

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38. Why do you need a Cookie

    Cookies are used to remember information about the user profile(such as username). It basically involves two steps,

    1. When a user visits a web page, the user profile can be stored in a cookie.
    2. Next time the user visits the page, the cookie remembers the user profile.

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39. What are the options in a cookie

    There are few below options available for a cookie,

    1. By default, the cookie is deleted when the browser is closed but you can change this behavior by setting expiry date (in UTC time).

    document.cookie = "username=John; expires=Sat, 8 Jun 2019 12:00:00 UTC";
    

    1. By default, the cookie belongs to a current page. But you can tell the browser what path the cookie belongs to using a path parameter.

    document.cookie = "username=John; path=/services";
    

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40. How do you delete a cookie

    You can delete a cookie by setting the expiry date as a passed date. You don't need to specify a cookie value in this case. For example, you can delete a username cookie in the current page as below.

    document.cookie = "username=; expires=Fri, 07 Jun 2019 00:00:00 UTC; path=/;";
    

    Note: You should define the cookie path option to ensure that you delete the right cookie. Some browsers doesn't allow to delete a cookie unless you specify a path parameter.

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41. What are the differences between cookie, local storage and session storage

    Below are some of the differences between cookie, local storage and session storage,

    | Feature | Cookie | Local storage | Session storage | |---- | --------- | ----- | ----- | | Accessed on client or server side | Both server-side & client-side | client-side only | client-side only | | Lifetime | As configured using Expires option | until deleted | until tab is closed | | SSL support | Supported | Not supported | Not supported | | Maximum data size | 4KB | 5 MB | 5MB |

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42. What is the main difference between localStorage and sessionStorage

    LocalStorage is the same as SessionStorage but it persists the data even when the browser is closed and reopened(i.e it has no expiration time) whereas in sessionStorage data gets cleared when the page session ends.

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43. How do you access web storage

    The Window object implements the WindowLocalStorage and WindowSessionStorage objects which has localStorage(window.localStorage) and sessionStorage(window.sessionStorage) properties respectively. These properties create an instance of the Storage object, through which data items can be set, retrieved and removed for a specific domain and storage type (session or local). For example, you can read and write on local storage objects as below

    localStorage.setItem('logo', document.getElementById('logo').value);
    localStorage.getItem('logo');
    

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44. What are the methods available on session storage

    The session storage provided methods for reading, writing and clearing the session data

    // Save data to sessionStorage
    sessionStorage.setItem('key', 'value');
    
    // Get saved data from sessionStorage
    let data = sessionStorage.getItem('key');
    
    // Remove saved data from sessionStorage
    sessionStorage.removeItem('key');
    
    // Remove all saved data from sessionStorage
    sessionStorage.clear();
    

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45. What is a storage event and its event handler

    The StorageEvent is an event that fires when a storage area has been changed in the context of another document. Whereas onstorage property is an EventHandler for processing storage events. The syntax would be as below

     window.onstorage = functionRef;
    

    Let's take the example usage of onstorage event handler which logs the storage key and it's values

    window.onstorage = function(e) {
      console.log('The ' + e.key +
        ' key has been changed from ' + e.oldValue +
        ' to ' + e.newValue + '.');
    };
    

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46. Why do you need web storage

    Web storage is more secure, and large amounts of data can be stored locally, without affecting website performance. Also, the information is never transferred to the server. Hence this is a more recommended approach than Cookies.

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47. How do you check web storage browser support

    You need to check browser support for localStorage and sessionStorage before using web storage,

    if (typeof(Storage) !== "undefined") {
      // Code for localStorage/sessionStorage.
    } else {
      // Sorry! No Web Storage support..
    }
    

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48. How do you check web workers browser support

    You need to check browser support for web workers before using it

    if (typeof(Worker) !== "undefined") {
      // code for Web worker support.
    } else {
      // Sorry! No Web Worker support..
    }
    

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49. Give an example of a web worker

    You need to follow below steps to start using web workers for counting example

    1. Create a Web Worker File: You need to write a script to increment the count value. Let's name it as counter.js

    let i = 0;
    
    function timedCount() {
      i = i + 1;
      postMessage(i);
      setTimeout("timedCount()",500);
    }
    
    timedCount();
    

    Here postMessage() method is used to post a message back to the HTML page

    1. Create a Web Worker Object: You can create a web worker object by checking for browser support. Let's name this file as web_worker_example.js

    if (typeof(w) == "undefined") {
      w = new Worker("counter.js");
    }
    

    and we can receive messages from web worker

    w.onmessage = function(event){
      document.getElementById("message").innerHTML = event.data;
    };
    

    1. Terminate a Web Worker: Web workers will continue to listen for messages (even after the external script is finished) until it is terminated. You can use the terminate() method to terminate listening to the messages.

    w.terminate();
    

    1. Reuse the Web Worker: If you set the worker variable to undefined you can reuse the code

    w = undefined;
    

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50. What are the restrictions of web workers on DOM

    WebWorkers don't have access to below javascript objects since they are defined in an external files

    1. Window object
    2. Document object
    3. Parent object

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51. What is a promise

    A promise is an object that may produce a single value some time in the future with either a resolved value or a reason that it’s not resolved(for example, network error). It will be in one of the 3 possible states: fulfilled, rejected, or pending.

    The syntax of Promise creation looks like below,

        const promise = new Promise(function(resolve, reject) {
          // promise description
        })
    

    The usage of a promise would be as below,

    const promise = new Promise(resolve => {
      setTimeout(() => {
        resolve("I'm a Promise!");
      }, 5000);
    }, reject => {
    
    });
    
    promise.then(value => console.log(value));
    

    The action flow of a promise will be as below,

    

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52. Why do you need a promise

    Promises are used to handle asynchronous operations. They provide an alternative approach for callbacks by reducing the callback hell and writing the cleaner code.

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53. What are the three states of promise

    Promises have three states:

    1. Pending: This is an initial state of the Promise before an operation begins
    2. Fulfilled: This state indicates that the specified operation was completed.
    3. Rejected: This state indicates that the operation did not complete. In this case an error value will be thrown.

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54. What is a callback function

    A callback function is a function passed into another function as an argument. This function is invoked inside the outer function to complete an action. Let's take a simple example of how to use callback function

    function callbackFunction(name) {
      console.log('Hello ' + name);
    }
    
    function outerFunction(callback) {
      let name = prompt('Please enter your name.');
      callback(name);
    }
    
    outerFunction(callbackFunction);
    

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55. Why do we need callbacks

    The callbacks are needed because javascript is an event driven language. That means instead of waiting for a response javascript will keep executing while listening for other events. Let's take an example with the first function invoking an API call(simulated by setTimeout) and the next function which logs the message.

    function firstFunction(){
      // Simulate a code delay
      setTimeout( function(){
        console.log('First function called');
      }, 1000 );
    }
    function secondFunction(){
      console.log('Second function called');
    }
    firstFunction();
    secondFunction();
    
    Output
    // Second function called
    // First function called
    

    As observed from the output, javascript didn't wait for the response of the first function and the remaining code block got executed. So callbacks are used in a way to make sure that certain code doesn’t execute until the other code finishes execution.

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56. What is a callback hell

    Callback Hell is an anti-pattern with multiple nested callbacks which makes code hard to read and debug when dealing with asynchronous logic. The callback hell looks like below,

    async1(function(){
        async2(function(){
            async3(function(){
                async4(function(){
                    ....
                });
            });
        });
    });
    

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57. What are server-sent events

    Server-sent events (SSE) is a server push technology enabling a browser to receive automatic updates from a server via HTTP connection without resorting to polling. These are a one way communications channel - events flow from server to client only. This has been used in Facebook/Twitter updates, stock price updates, news feeds etc.

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58. How do you receive server-sent event notifications

    The EventSource object is used to receive server-sent event notifications. For example, you can receive messages from server as below,

    if(typeof(EventSource) !== "undefined") {
      var source = new EventSource("sse_generator.js");
      source.onmessage = function(event) {
        document.getElementById("output").innerHTML += event.data + "<br>";
      };
    }
    

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59. How do you check browser support for server-sent events

    You can perform browser support for server-sent events before using it as below,

    if(typeof(EventSource) !== "undefined") {
      // Server-sent events supported. Let's have some code here!
    } else {
      // No server-sent events supported
    }
    

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60. What are the events available for server sent events

    Below are the list of events available for server sent events | Event | Description | |---- | --------- | onopen | It is used when a connection to the server is opened | | onmessage | This event is used when a message is received | | onerror | It happens when an error occurs|

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61. What are the main rules of promise

    A promise must follow a specific set of rules,

    1. A promise is an object that supplies a standard-compliant .then() method
    2. A pending promise may transition into either fulfilled or rejected state
    3. A fulfilled or rejected promise is settled and it must not transition into any other state.
    4. Once a promise is settled, the value must not change.

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62. What is callback in callback

    You can nest one callback inside in another callback to execute the actions sequentially one by one. This is known as callbacks in callbacks.

    loadScript('/script1.js', function(script) {
       console.log('first script is loaded');
    
      loadScript('/script2.js', function(script) {
    
        console.log('second script is loaded');
    
        loadScript('/script3.js', function(script) {
    
            console.log('third script is loaded');
          // after all scripts are loaded
        });
    
      })
    
    });
    

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63. What is promise chaining

    The process of executing a sequence of asynchronous tasks one after another using promises is known as Promise chaining. Let's take an example of promise chaining for calculating the final result,

    new Promise(function(resolve, reject) {
    
      setTimeout(() => resolve(1), 1000);
    
    }).then(function(result) {
    
      console.log(result); // 1
      return result * 2;
    
    }).then(function(result) {
    
      console.log(result); // 2
      return result * 3;
    
    }).then(function(result) {
    
      console.log(result); // 6
      return result * 4;
    
    });
    

    In the above handlers, the result is passed to the chain of .then() handlers with the below work flow,

    1. The initial promise resolves in 1 second,
    2. After that .then handler is called by logging the result(1) and then return a promise with the value of result * 2.
    3. After that the value passed to the next .then handler by logging the result(2) and return a promise with result * 3.
    4. Finally the value passed to the last .then handler by logging the result(6) and return a promise with result * 4.

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64. What is promise.all

    Promise.all is a promise that takes an array of promises as an input (an iterable), and it gets resolved when all the promises get resolved or any one of them gets rejected. For example, the syntax of promise.all method is below,

    Promise.all([Promise1, Promise2, Promise3]) .then(result) => {   console.log(result) }) .catch(error => console.log(`Error in promises ${error}`))
    

    Note: Remember that the order of the promises(output the result) is maintained as per input order.

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65. What is the purpose of the race method in promise

    Promise.race() method will return the promise instance which is firstly resolved or rejected. Let's take an example of race() method where promise2 is resolved first

    var promise1 = new Promise(function(resolve, reject) {
        setTimeout(resolve, 500, 'one');
    });
    var promise2 = new Promise(function(resolve, reject) {
        setTimeout(resolve, 100, 'two');
    });
    
    Promise.race([promise1, promise2]).then(function(value) {
      console.log(value); // "two" // Both promises will resolve, but promise2 is faster
    });
    

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66. What is a strict mode in javascript

    Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. This way it prevents certain actions from being taken and throws more exceptions. The literal expression "use strict"; instructs the browser to use the javascript code in the Strict mode.

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67. Why do you need strict mode

    Strict mode is useful to write "secure" JavaScript by notifying "bad syntax" into real errors. For example, it eliminates accidentally creating a global variable by throwing an error and also throws an error for assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object.

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68. How do you declare strict mode

    The strict mode is declared by adding "use strict"; to the beginning of a script or a function. If declared at the beginning of a script, it has global scope.

    "use strict";
    x = 3.14; // This will cause an error because x is not declared
    

    and if you declare inside a function, it has local scope

    x = 3.14;       // This will not cause an error.
    myFunction();
    
    function myFunction() {
      "use strict";
      y = 3.14;   // This will cause an error
    }
    

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69. What is the purpose of double exclamation

    The double exclamation or negation(!!) ensures the resulting type is a boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, true. For example, you can test IE version using this expression as below,

    let isIE8 = false;
    isIE8 = !! navigator.userAgent.match(/MSIE 8.0/);
    console.log(isIE8); // returns true or false
    

    If you don't use this expression then it returns the original value.

    console.log(navigator.userAgent.match(/MSIE 8.0/));  // returns either an Array or null
    

    Note: The expression !! is not an operator, but it is just twice of ! operator.

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70. What is the purpose of the delete operator

    The delete keyword is used to delete the property as well as its value.

    var user= {name: "John", age:20};
    delete user.age;
    
    console.log(user); // {name: "John"}
    

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71. What is the typeof operator

    You can use the JavaScript typeof operator to find the type of a JavaScript variable. It returns the type of a variable or an expression.

    typeof "John Abraham"     // Returns "string"
    typeof (1 + 2)        // Returns "number"
    

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72. What is undefined property

    The undefined property indicates that a variable has not been assigned a value, or not declared at all. The type of undefined value is undefined too.

    var user;    // Value is undefined, type is undefined
    console.log(typeof(user)) //undefined
    

    Any variable can be emptied by setting the value to undefined.

    user = undefined
    

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73. What is null value

    The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values. The type of null value is object. You can empty the variable by setting the value to null.

    var user = null;
    console.log(typeof(user)) //object
    

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74. What is the difference between null and undefined

    Below are the main differences between null and undefined,

    | Null | Undefined | |---- | -----------| | It is an assignment value which indicates that variable points to no object. | It is not an assignment value where a variable has been declared but has not yet been assigned a value. | | Type of null is object | Type of undefined is undefined | | The null value is a primitive value that represents the null, empty, or non-existent reference. | The undefined value is a primitive value used when a variable has not been assigned a value.| | Indicates the absence of a value for a variable | Indicates absence of variable itself | | Converted to zero (0) while performing primitive operations | Converted to NaN while performing primitive operations |

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75. What is eval

    The eval() function evaluates JavaScript code represented as a string. The string can be a JavaScript expression, variable, statement, or sequence of statements.

    console.log(eval('1 + 2')); //  3
    

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76. What is the difference between window and document

    Below are the main differences between window and document,

    | Window | Document | |---- | --------- | It is the root level element in any web page | It is the direct child of the window object. This is also known as Document Object Model(DOM) | | By default window object is available implicitly in the page | You can access it via window.document or document. | | It has methods like alert(), confirm() and properties like document, location | It provides methods like getElementById, getElementByTagName, createElement etc |

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77. How do you access history in javascript

    The window.history object contains the browser's history. You can load previous and next URLs in the history using back() and next() methods.

    function goBack() {
      window.history.back()
    }
    function goForward() {
      window.history.forward()
    }
    

    Note: You can also access history without window prefix.

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78. What are the javascript data types

    Below are the list of javascript data types available

    1. Number
    2. String
    3. Boolean
    4. Object
    5. Undefined

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79. What is isNaN

    The isNaN() function is used to determine whether a value is an illegal number (Not-a-Number) or not. i.e, This function returns true if the value equates to NaN. Otherwise it returns false.

    isNaN('Hello') //true
    isNaN('100') //false
    

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80. What are the differences between undeclared and undefined variables

    Below are the major differences between undeclared and undefined variables,

    | undeclared | undefined | |---- | --------- | These variables do not exist in a program and are not declared | These variables declared in the program but have not assigned any value | | If you try to read the value of an undeclared variable, then a runtime error is encountered | If you try to read the value of an undefined variable, an undefined value is returned. |

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81. What are global variables

    Global variables are those that are available throughout the length of the code without any scope. The var keyword is used to declare a local variable but if you omit it then it will become global variable

    msg = "Hello" // var is missing, it becomes global variable
    

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82. What are the problems with global variables

    The problem with global variables is the conflict of variable names of local and global scope. It is also difficult to debug and test the code that relies on global variables.

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83. What is NaN property

    The NaN property is a global property that represents "Not-a-Number" value. i.e, It indicates that a value is not a legal number. It is very rare to use NaN in a program but it can be used as return value for few cases

    Math.sqrt(-1)
    parseInt("Hello")
    

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84. What is the purpose of isFinite function

    The isFinite() function is used to determine whether a number is a finite, legal number. It returns false if the value is +infinity, -infinity, or NaN (Not-a-Number), otherwise it returns true.

    isFinite(Infinity);  // false
    isFinite(NaN);       // false
    isFinite(-Infinity); // false
    
    isFinite(100);         // true
    

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85. What is an event flow

    Event flow is the order in which event is received on the web page. When you click an element that is nested in various other elements, before your click actually reaches its destination, or target element, it must trigger the click event for each of its parent elements first, starting at the top with the global window object. There are two ways of event flow

    1. Top to Bottom(Event Capturing)
    2. Bottom to Top (Event Bubbling)

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86. What is event bubbling

    Event bubbling is a type of event propagation where the event first triggers on the innermost target element, and then successively triggers on the ancestors (parents) of the target element in the same nesting hierarchy till it reaches the outermost DOM element.

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87. What is event capturing

    Event capturing is a type of event propagation where the event is first captured by the outermost element, and then successively triggers on the descendants (children) of the target element in the same nesting hierarchy till it reaches the innermost DOM element.

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88. How do you submit a form using JavaScript

    You can submit a form using JavaScript use document.form[0].submit(). All the form input's information is submitted using onsubmit event handler

    function submit() {
        document.form[0].submit();
    }
    

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89. How do you find operating system details

    The window.navigator object contains information about the visitor's browser OS details. Some of the OS properties are available under platform property,

    console.log(navigator.platform);
    

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90. What is the difference between document load and DOMContentLoaded events

    The DOMContentLoaded event is fired when the initial HTML document has been completely loaded and parsed, without waiting for assets(stylesheets, images, and subframes) to finish loading. Whereas The load event is fired when the whole page has loaded, including all dependent resources(stylesheets, images).

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91. What is the difference between native, host and user objects

    Native objects are objects that are part of the JavaScript language defined by the ECMAScript specification. For example, String, Math, RegExp, Object, Function etc core objects defined in the ECMAScript spec. Host objects are objects provided by the browser or runtime environment (Node). For example, window, XmlHttpRequest, DOM nodes etc are considered as host objects. User objects are objects defined in the javascript code. For example, User objects created for profile information.

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92. What are the tools or techniques used for debugging JavaScript code

    You can use below tools or techniques for debugging javascript

    1. Chrome Devtools
    2. debugger statement
    3. Good old console.log statement

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93. What are the pros and cons of promises over callbacks

    Below are the list of pros and cons of promises over callbacks,

    Pros:

    1. It avoids callback hell which is unreadable
    2. Easy to write sequential asynchronous code with .then()
    3. Easy to write parallel asynchronous code with Promise.all()
    4. Solves some of the common problems of callbacks(call the callback too late, too early, many times and swallow errors/exceptions)

    Cons:

    1. It makes little complex code
    2. You need to load a polyfill if ES6 is not supported

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94. What is the difference between an attribute and a property

    Attributes are defined on the HTML markup whereas properties are defined on the DOM. For example, the below HTML element has 2 attributes type and value,

    <input type="text" value="Name:">
    

    You can retrieve the attribute value as below,

    const input = document.querySelector('input');
    console.log(input.getAttribute('value')); // Good morning
    console.log(input.value); // Good morning
    

    And after you change the value of the text field to "Good evening", it becomes like

    console.log(input.getAttribute('value')); // Good morning
    console.log(input.value); // Good evening
    

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95. What is same-origin policy

    The same-origin policy is a policy that prevents JavaScript from making requests across domain boundaries. An origin is defined as a combination of URI scheme, hostname, and port number. If you enable this policy then it prevents a malicious script on one page from obtaining access to sensitive data on another web page using Document Object Model(DOM).

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96. What is the purpose of void 0

    Void(0) is used to prevent the page from refreshing. This will be helpful to eliminate the unwanted side-effect, because it will return the undefined primitive value. It is commonly used for HTML documents that use href="JavaScript:Void(0);" within an <a> element. i.e, when you click a link, the browser loads a new page or refreshes the same page. But this behavior will be prevented using this expression. For example, the below link notify the message without reloading the page

    <a href="JavaScript:void(0);" onclick="alert('Well done!')">Click Me!</a>
    

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97. Is JavaScript a compiled or interpreted language

    JavaScript is an interpreted language, not a compiled language. An interpreter in the browser reads over the JavaScript code, interprets each line, and runs it. Nowadays modern browsers use a technology known as Just-In-Time (JIT) compilation, which compiles JavaScript to executable bytecode just as it is about to run.

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98. Is JavaScript a case-sensitive language

    Yes, JavaScript is a case sensitive language. The language keywords, variables, function & object names, and any other identifiers must always be typed with a consistent capitalization of letters.

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99. Is there any relation between Java and JavaScript

    No, they are entirely two different programming languages and have nothing to do with each other. But both of them are Object Oriented Programming languages and like many other languages, they follow similar syntax for basic features(if, else, for, switch, break, continue etc).

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100. What are events

     Events are "things" that happen to HTML elements. When JavaScript is used in HTML pages, JavaScript can react on these events. Some of the examples of HTML events are,

     1. Web page has finished loading
     2. Input field was changed
     3. Button was clicked

     Let's describe the behavior of click event for button element,

     <!doctype html>
     <html>
      <head>
        <script>
          function greeting() {
            alert('Hello! Good morning');
          }
        </script>
      </head>
      <body>
        <button type="button" onclick="greeting()">Click me</button>
      </body>
     </html>
     

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