Frequently Asked JavaScript Interview Questions And Answers For Developer

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Latest Frequently Asked Javascript interview questions of 2020

The Frequently asked JavaScript interview questions with answers for beginners are given below.

Q1. What are the difference between == and === in java script?

== Check contents are equal or not of both values.

=== Check contents as well as type of both values.

Ex:

3==='3'//false, because one is integer and other is string value.

3==="3"//false, because one is integer and other is string value.

"3"==3//true, because both have same value i.e. 3

 3===3// true, because both have same value i.e. 3

Q2. What is !! (Double Not-Operator) in Java script?

!! converts a value (null, undefined, objects etc…) to a primitive Boolean value(true/false).

Ex: alert (!!undefined); //false

alert (!!null); //false

alert (!!NaN); //false

alert (!!1); //true

alert(!!'test'); //true

alert (!!-1); //true

Q3. What is Void 0 in Javascript?

void 0 returns undefined. void 0 and undefined both are working  same.

For older browser, undefined is used. In modern browsers (which supports javascript 1.8.5) we use void 0.

var myVal = 1;

alert(myVal);  //1

void 0 is safer and can be used in place of undefined.

Other advantage of void 0 is less type than undefined.

var myVar;

alert(myVar === void 0); //true

Q4. What happens when a browser starts loading a web page?

1. At first browser starts parsing the HTML contents.
2. When the parser encounters a <script> tag that references an external JavaScript file. The parser stops parsing the HTML and the browser makes a request to download the script file. Until the download is complete, the parser is blocked from parsing the rest of the HTML on the page.
3. When the download is complete, that's when the parser will resume to parse the rest of the HTML. 
This means the page loading is stopped until all the scripts are loaded which affects user experience.

Q5. What are the different methods available in JavaScript to convert strings to numbers.

Following functions are used to convert strings to numbers:
1. parseInt()  2. parseFloat() 3. isNan()

Q6. How to use parseInt() function?

function addNumbers()

{

    var firstNumber = parseInt("10");

    var secondNumber = parseInt("20");

    var c = firstNumber + secondNumber;//Output is 30}
         

Q7. What is the output of parseInt (“20.5”)+ parseInt (“10.3”)?

Output is 30, decimal part is ignored.

Q8. What is the use of parseFloat() function?

To retain the decimal places, use parseFloat() function.

function addNumbers()

{

    var firstNumber = parseFloat(“20.5”);

    var secondNumber = parseFloat(“10.3");

   var c= firstNumber + secondNumber;//Output is 30.8

}

Q9. What is NaN & isNaN() function in Javascript?

NaN in JavaScript stands for Not-a-Number. In JavaScript we have isNaN() function which determines whether a value is an illegal number. This function returns true if the value is not a number, and false if not.

function addNumbers()

{

    var firstNumber = parseFloat(document.getElementById("txtFirstNumber").value);

    if (isNaN(firstNumber))

    {

alert("Please enter a valid number in the first number textbox");

        return;

    }

    var secondNumber = parseFloat(document.getElementById("txtSecondNumber").value);

    if (isNaN(secondNumber))

    {

alert("Please enter a valid number in the second number textbox");

        return;

    }

    document.getElementById("txtResult").value = firstNumber + secondNumber;

}

Now, when you leave first number and second number textboxes blank or if you enter text instead of a number, and when you click the Add button, you get validation error messages as expected.

Q10. How to make the validation error message more relevant?

We are better understand the validation error message by below example.

function addNumbers()

{  var firstNumber = document.getElementById("txtOneValue").value;

    var secondNumber = document.getElementById("txtTwoValue").value;

    if (firstNumber == "")

    { alert("First Number is required");   return;  }

    firstNumber = parseFloat(firstNumber);

    if (isNaN(firstNumber))

    { alert("Please enter a valid number in the first textbox");  return; }

    if (secondNumber == "")

    { alert("Second Number is required");  return; }

    secondNumber = parseFloat(secondNumber)

    if (isNaN(secondNumber))

    { alert("Please enter a valid number in the second textbox");   return;  }

    document.getElementById("txtResult").value = firstNumber + secondNumber;

}

Q11. What is the output of below statements?

var string1="string in double quotes"  var string2 = 'string in single quotes'

Both string1 and string2 performing same output. You can use either single or double quotes in a single statement.

Q12. How to Concatenating strings in JavaScript?

There are 2 options either use + operator or concat() method.
using + operator:

var string1 = "Hello"  var string2 = "JavaScript"

var result = string1 + " " + string2;

alert(result); Output : Hello JavaScript

using concat() method:

var string1 = "Hello"  var string2 = "JavaScript"

var result = string1.concat(" ", string2);

alert(result);  Output : Hello JavaScript

Q13. How to use single quotes inside a string in Java script?

There are 2 options:

Option 1: Use single quotes inside the string wherever you need them.

Example : var myString = "Welcome to 'JavaScript' Training";

alert(myString); 

Output : Welcome to 'JavaScript' Training

Option 2: Use escape sequence character \ with a single quote inside the string.

Example : var myString = 'Welcome to \'JavaScript\' Training';

alert(myString);

Output : Welcome to 'JavaScript' Training

Q14. How to use double quotes inside a string in Java script?

There are 2 options:

Option 1: Use double quotes inside the string wherever you need them.

Example : var myString = ‘Welcome to “JavaScript” Training’;

alert(myString);  Output : Welcome to “JavaScript” Training

Option 2: Use escape sequence character \ with a single quote inside the string.

Example : var myString = “Welcome to \”JavaScript\” Training”;

alert(myString); Output : Welcome to “JavaScript” Training

Q15. How to Converting a string to uppercase in JavaScript?

Use toUpperCase() function:
Example : var upperCaseString = "JavaScript";

alert(upperCaseString.toUpperCase());

Output : JAVASCRIPT

Q16. How to Converting a string to lowercase in JavaScript? 

Use toLowerCase() function:
Example :var lowerCaseString = "JavaScript";

alert(lowerCaseString.toLowerCase());

Output : javascript

Q17. How to find Length of string in JavaScript?

Use length property:
Example : alert("JavaScript".length); 

Output : 10

Example :var myString = "Hello JavaScript";

alert(myString.length); 

Output : 16

Q18. How to remove whitespace from both ends of a string in JavaScript?

Use trim() function:
Example :

var string1 = " AB ";

var string2 = " CD ";

var result = string1.trim() + string2.trim();

alert(result); 

Output : ABCD

Q19. How to Replace strings in JavaScript?

Use replace() method:

This method searches a given string for a specified value or a regular expression, replaces the specified values with the replacement values and returns a new string. This method does not change the original string.
Example: Replaces JavaScript with World

var myString = "Hello JavaScript";

var result = myString.replace("JavaScript", "World");

alert(result);

Output : Hello World

Q20. How to perform a case-sensitive global replacement in JavaScript?

Using a regular expression between the 2 forward slashes (//). The letter g after the forward slash specifies a global replacement. The match here is case sensitive. This means Red(with capital R) is not replaced with green.

var myString = "A Red bottle with a red liquid is on a red table";

var result = myString.replace(/red/g, "green");

//g means global replacement with case sensitive value

alert(result); 

Output : A Red bottle with a green liquid is on a green table

Q21. How to Perform a case-insensitive global replacement in JavaScript?

The letters gi after the forward slash indicates to do global case-insensitive replacement. Notice that the word Red (with capital R) is also replaced with green.
var myString = "A Red bottle with a red liquid is on a red table";

var result = myString.replace(/red/gi, "green");

alert(result); 

Output : A green bottle with a green liquid is on a green table 

Final words

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