Best Javascript Interview Questions For Front End Developer

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Latest Best Javascript interview questions of 2020

The Best JavaScript interview questions with answers for Front End Developer are given below.

Q1. What is function in java script?

A JavaScript function is a block of code. This function perform a specific task.

This function executes when "something" invokes it or calls it.

Ex:

function myFunc(a, b) {

  return a + b;   // The function returns the sum of a and b.

}

Q2. How code executes in a function in java script?

The code executes, in a function, is placed inside curly brackets: {}

function myFunc(param1, param2, ……) {

  // code to be executed

}

Note: Inside the function, parameters behave as local variables.

Q3. Why Functions in Java script?

Code reusability: Define the code once, and use it many times.

You can use the same code many times with different arguments, to produce different results.

Q4. When we invoke function in JavaScript?

We invoke function in JavaScript when:

An event occurs (button click, mouse up, mouse down, etc.…)

• Called from a JavaScript code.
• Automatically/ self-invoked

Q5. What is return statement in a function in java script?

When function reaches the return statement, the function will stop executing.

At this point functions compute a return value. The return value is returned back to the caller.

Example: Calculate the sum of two numbers, and return the result:

var myVal = sumFunction(4, 3);   // sumFunction is called, return value will end up in myVar.

function sumFunction (x, y) {

  return x +y;             // Function returns the sum of x and y

}

The result in x will be: 7

Q6. What are objects in a JavaScript?

Objects are collection of values. Each value can be written as name: value pairs.

Ex: var person = {firstName:"John", lastName:"Doe", age: 33};

Q7. What are Properties in java script?

The name: values pairs in JavaScript objects are called properties:

Property       Property Value

firstName     John

lastName     Doe

age              33

Q8. How to Accessing Object Properties in java script?

You can access object properties in two ways:

objectName.propertyName        or       objectName["propertyName"]

Example1: person.lastName;

Example2: person["lastName"];

Q9. What is Object Methods in Java script?

A method is a function stored as a property.

Example

var person = {

          firstName: "John",

          lastName : "Doe",

                   id   : 5566,

          fullName : function() {

    return this.firstName + " " + this.lastName;

  }

};

Q10. What is this Keyword in a function in Java script?

In a function, this refers to the "owner" of the function.

In the above example, this is the person object that "owns" the fullName function.

Q11. How to access Object Methods in java script?

You can access an object method by following syntax: objectname.methodname()

Example: name = person.fullName();

Q12. What is events in Java Script?

An HTML event is an activity done by the browser, or done by a user.

Ex:

Events	Description
onclick	occurs when HTML element is clicked.
onfocus	occurs when an HTML element gets focus.
onblur	occurs when HTML form loses the focus from an element.
onsubmit	occurs when HTML form is submitted.
Onmouseover	occurs when mouse is moved over an HTML element.
onmouseout	occurs when mouse is moved out from an HTML element.
onmousedown	occurs when mouse button is pressed over an html element.
onmouseup	occurs when mouse is released from an html element.
onload	occurs when document, frameset is loaded.
onunload	occurs when body or frameset is unloaded.
onscroll	occurs when document is scrolled.
onresized	occurs when document is resized.
onreset	occurs when form is reset.
onkeydown	occurs when key is being pressed.
onkeypress	occurs when user presses the key.
onkeyup	occurs when key is released.

Q13. How event handler attributes used in Java script?

We used it in two ways:

With single quotes: <element event='some JavaScript'>

Ex:

<button onclick=’displayDate()’>The date is?</button>

With double quotes:<element event="some JavaScript">

Ex:<button onclick="displayDate()">The date is?</button>

Q14. What is JavaScript string?

A JavaScript string is zero or more characters written inside single or double quotes.

In JavaScript strings are used for storing and manipulating text.

Ex:

var x = ‘John Doe’;

Ex:

var x = "John Doe";

Q15. How to retrieve a substring from a given string in Java Script?

The following 3 methods are used to retrieve a substring from a given string:

1.   substring()

2.   substr()

3.   slice()

substring() method : This method extracts the characters from a string. It always returns a new substring. This method has 2 parameters start and end. The character at the end position is not included in the substring.

Syntax: string.substring(start, end)

start argument is required.

end argument is optional.

Ex:

var str = "Hello India!";

Positions:

H(0 Position)

e(1^st position) and so on.

var res = str.substring(6);//Start from 6th character.

Output: India

Ex: Extract the first 10 characters

var str = "JavaScript World";

var result = str.substring(0, 10);// 0 to 9^th charachers

Output: JavaScript

Ex: If "start" >"end", it will swap the two arguments:

var str = "India!";

var res = str.substring(5,1);// Here 5 is greater than 1

Output: ndia

substr() method : This method has 2 parameters start and count. Start argument is required. Count argument is optional.

Extract the first 10 characters

var str = "JavaScript World";

var result = str.substr(0, 10);

Output : JavaScript

Ex:

var str = "JavaScript World";

var result = str.substr(11);

alert(result);

Output : World

slice() method : This method extracts the characters from a string. It always returns a new substring. This method has 2 parameters start and end. The character at the end position is not included in the substring.

Syntax: string.slice(start, end)

start argument is required.

end argument is optional.

Ex: Extract the first 10 characters

var str = "JavaScript World";

var result = str.slice(0, 10);

Output: JavaScript

Ex: If the end parameter is not specified.

var str = "JavaScript World";

var result = str.slice(11);

Output : World

Q16. What is the difference between substr and substring methods
Main difference:

1.   Second parameter of substring() method specifies the index position where as second parameter of substr() method specifies the number of characters to return.

2.   substr() method does not work in IE8 and earlier versions.

Ex: Extract the first 10 characters

var str = "JavaScript World";

var result = str.substring(0, 10);// 0 to 9^th charachers

Output: JavaScript

Ex: Extract the first 10 characters

var str = "JavaScript World";

var result = str.substr(0, 10);

Output: JavaScript

Q17. What is the difference between slice and substring?

If start parameter is greater than stop parameter, then substring will swap those 2 parameters, whereas slice will not swap.

Q18. Where we can use indexOf() and substring() methods?

function getEmailandDomainParts()

{   var emailAddress = document.getElementById("txtEmailAddress").value;

    var emailPart = emailAddress.substring(0, emailAddress.indexOf("@"));

    var domainPart = emailAddress.substring(emailAddress.indexOf("@") + 1);

    document.getElementById("txtEmailPart").value = emailPart;

    document.getElementById("txtDomainPart").value = domainPart;

}

Input: anmolg4u@gmail.com

Output: anmolg4u (Email Part) & gmail.com (domain part).

lastIndexOf() method returns the position of the last occurrence of a specified value in a string. Since it's job is to return the last index of the specified value, this method searches the given string from the end to the beginning and returns the index of the first match it finds. This method returns -1 if the specified value is not available in the given string.

Example: Retrieve the last index position of dot (.) in the given string

var url = "http://www.mytutorials.com";

alert(url.lastIndexOf(".")); Output : 22

Q19. Give me real time example where lastIndexOf and substring methods can be used?

Ex: Find the domain name from the url.

<script type="text/javascript">

    function getDomainName()

    {  var url = document.getElementById("txtURL").value;

        var domainName = url.substr(url.lastIndexOf("."));

        document.getElementById("txtDomian").value = domainName;

    }

</script>

Final words

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