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Latest Javascript Interview Questions & Answers

JavaScript is the true calling for self-driven and aficionado technocrats. The industry is creating a vast and remarkable job pool for both experienced and fresher professional. If you also have a right skill like a JavaScript course and training from a reputed institute like Koenig Solutions then you are all set to give a knock on copious opportunities that JavaScript offers.

Alongside, we are here to our rundown of top JavaScript Interview questions and answers. Read them out before appearing in an interview so that you can succeed:

  1. How window.onload is different from onDocumentReady?

They both function differently. While window.onload will take time to respond and don’t actually respond until the entire information is not loaded, on Document Ready is fast and accurate. It starts working just after the DOM gets uploaded.

  1. How will you change the style or class of any element through JavaScript? Is it possible?

Yes, it can be done using the simple commands.

For example, style of font (changing the font size to 16) could be changes as follows:

document.getElementById (“text_string”).style.fontSize = “16”; (For Style)

Similarly, class can be changed as:

document.getElementById(“text_string”).className = “New_Class”; ( For Class).

It will change the values while the web page loads.

  1. What would be the output of following code?
var y = 1;
   if (function f(){}) {
   y += typeof f;

Based upon the given commend, the out would be as:

var k = 1;
if (1) {
    eval(function foo(){});
    k += typeof foo;

The reason of this undefined output is the “IF” statement does the evaluation with the help of eval which returns function f () ().

  1. Write a function that will loop through a list of integers and print the index of each element after a 3-second delay.
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The correct function for this loop would be:

const arr = [10, 12, 15, 21];
for (var i = 0; i < arr.length; i++) {
// pass in the variable i so that each function
// has access to the correct index
setTimeout(function(i_local) {
return function() {
console.log('The index of this number is: ' + i_local);
}(i), 3000);


  1. What do you mean by “Debouncing” and what is the correct implementation of it?

Debouncing can be defined as the best possible way to resolves the issues by limiting the time lap by passing the function to action. The right way to implement debouncing is to club various function into one and execute them one after another once the time has elapsed for some time.

  1. How is throttling different from Debouncing?

Unlike debouncing, throttling doesn’t wait for some time to pass over before executing another function. It spread the function calls along a longer time interval. So, it is more prompt while operating. It can easily handle more than 10 events within minute time duration of milliseconds.

  1. What are the drawbacks of creating true private methods in JavaScript?

The major drawback of the private method is they are highly memory-insufficient.

  1. What do you mean by NaN? What is its type? How can you reliably test if a value is equal to NaN or not?
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NaN basically stands for “not a number”. It usually occurs when an operation can’t perform properly owing to two reasons:

  • When operands are non-numeric.
  • When the result of the said operation is non-numeric.

It also often get considered with false.

console.log (NaN === NaN);  // logs "false"

One can easily rely on whether a number is NaN or not by taking the aid of built-in function isNaN(. Another most viable option to test is to use value!== value. It will only generate true solution if the valve is equal to NaN.

  1. What do you know about JavaScript namespacing? Elaborate its implementation?

Well, JavaScript namespacing is the most trusted and preferable way to bundle up the entire function by giving it a unique name.  Its implementation in JavaScript is vast as it used to promote modularity and code reuse in the application.

  1. What are all data types supported in JavaScript?

JavaScript support a remarkable number of data types including number, undefined, string, null, and Boolean.

Michael WarneAbout the Author:

The Michael Warne is a JavaScript professional with more than 5 years of experience in the industry. The author has imparted JavaScript course training, to working professionals and has been proactively talking about the importance of this course.

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2 Responses to “Latest Javascript Interview Questions & Answers”

  1. pooja says:

    great information, these questions very helpful in the interview. thank you for sharing and keep posting

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