JavaScript has evolved from being a programming language being used for interactive and dynamic web pages or web sites to development of mobile and web applications using the various JavaScript frameworks like Angular, React, etc.

JavaScript has seen tremendous growth is usage across the industry and has become the most sought after programming language. We have listed the top interview questions to enable you to clear JavaScript interview in a breeze.

Q.1 What are the various data types present in the JavaScript programming language?
The data types in the JavaScript programming language are: Number, String, Boolean, Object and Undefined
Q.2 Illustrate the utility of isNaN function in JavaScript
The isNaN function in JavaScript returns a Boolean value of true if the argument is not a number data type else false is returned by the function.
Q.3 Describe negative Infinity in the JavaScript programming language?
Negative Infinity is a number in JavaScript which can be derived by dividing negative number by zero.
Q.4 Which character is used to break JavaScript Code spanning into many lines?
The backslash, '\,'character is used at the end of the first line if the JavaScript code spans multiple lines.
Q.5 Describe undeclared and undefined variables in the JavaScript programming language?
Undeclared variables are those that do not exist in a program and are not declared. If the program tries to read the value of an undeclared variable, then a runtime error is encountered. Undefined variables are those that are declared in the program but have not been given any value. If the program tries to read the value of an undefined variable, an undefined value is returned.
Q.6 Explain global variables in the JavaScript programming language?
Global variables are available throughout the length of the code so that it has no scope. If the var keyword is omitted in declaring a variable, a global variable is declared.
Q.7 What does prompt box refers to in JavaScript
A prompt box is a box that allows the user to enter input by providing a text box. A label and box will be provided to enter the text or number.
Q.8 What does the 'this' keyword refers to, in JavaScript
'This' keyword refers to the object from where it was called.
Q.9 Explain the utility of timers in JavaScript
Timers are used to execute a piece of code at a set time or repeat the code in a given interval. This is done by using the functions setTimeout, setInterval, and clearInterval.
Q.10 Which character is used to denote a comment in JavaScript
The // character for Single line comments and /* */ characters for Multi Line comments
Q.11 A ViewState, is
'ViewState' is specific to a page in a session.
Q.12 What refers to SessionState
'SessionState' is specific to user-specific data that can be accessed across all web application pages.
Q.13 What is the use of the === operator in the JavaScript programming language?
=== is called a strict equality operator, which returns true when the two operands have the same value without conversion.
Q.14 Confirm if automatic type conversion is supported by the JavaScript programming language
Yes, JavaScript supports automatic type conversion.
Q.15 Describe Variable typing in Javascript language
Variable typing is used to assign a number to a variable. The same variable can be assigned to a string.
Q.16 Which function is used to convert the string of any base to an integer in JavaScript language
The parseInt() function is used to convert numbers between different bases.
Q.17 What is the difference between "==" and "===" in JavaScript language
== checks only for equality in value, whereas "===" is a stricter equality test and returns false if either the value or the type of the two variables are different.
Q.18 What would be the result of 3+2+"7" in the JavaScript programming language
The result is;’ 57 (as it evaluates to 3 + 2 and concatenation with 7 as, 5+”7” ).
Q.19 What will list the operating system on the client machine by JavaScript
The navigator. Platform string (property) is used in JavaScript.
Q.20 What does NULL in Javascript language, refers to
The NULL value is used to represent no value or no object. It implies no object or null string, no valid boolean value, no number, and no array object.
Q.21 What is the usage of delete operator in the JavaScript programming language
The delete keyword is used to delete the property as well as its value.
Q.22 Explain undefined value in JavaScript language
Undefined value means the: variable used in the code doesn't exist or Variable is not assigned to any value or Property does not exist.
Q.23 List all Pop up box types in JavaScript language
The popup boxes available in JavaScript are as: Alert, Confirm and Prompt
Q.24 Where does Void (0) is used in JavaScript language
Void(0) is used to prevent the page from refreshing, and parameter "zero" is passed while calling. Void(0) is used to call another method without refreshing the page.
Q.25 List the data type of variables in JavaScript language
All variables in JavaScript are object data types.
Q.26 Differentiate amongst an alert box and a confirmation box
An alert box displays only one button, which is the OK button. But a Confirmation box displays two buttons, namely OK and cancel.
Q.27 Describe escape characters in JavaScript language
Escape characters (Backslash) is used when working with special characters like single quotes, double quotes, apostrophes, and ampersands. Place backslash before the characters to make it display.
Q.28 Explain JavaScript Cookies
Cookies are the small test files stored in a computer, and they get created when the user visits the websites to store information that they need. Examples could be User Name details and shopping cart information from previous visits.
Q.29 Describe the usage of pop()method in JavaScript language
The pop() method is similar to the shift() method, but the difference is that the Shift method works at the array's start. The pop() method takes the last element off of the given array and returns it. The array on which it is called is then altered.
Q.30 Differentiate between the break and continue statement in JavaScript language
Break statement exits from the current loop. Continue statement continues with next statement of the loop.
Q.31 Generic objects in JavaScript language can be created by
Generic objects can be created as: var I = new object();
Q.32 The ‘type of’ operator in JavaScript language is used to
'Typeof' is an operator used to return a string description of the type of a variable.
Q.33 List the keywords in JavaScript language for exception handling
Try… Catch---finally is used to handle exceptions in the JavaScript
Q.34 Which keyword is used to print the text on the screen
Document. Write ("Welcome") is used to print the text–Welcome on the screen.
Q.35 What is the use of the blur function
Blur function is used to remove the focus from the specified object.
Q.36 What are the different types of errors in JavaScript
There are three types of errors: Load time errors(Errors that come up when loading a web page, like improper syntax errors, are known as Load time errors and generate the errors dynamically.), Runtime errors (Errors that come due to misuse of the command inside the HTML language.) and Logical Errors (These are the errors that occur due to the bad logic performed on a function with a different operation.)
Q.37 What is the use of the Push method in JavaScript
The push method is used to add or append one or more elements to an Array end. Using this method, we can append multiple elements by passing multiple arguments.
Q.38 What is the unshift method in JavaScript
Unshift method is like the push method, which works at the beginning of the array. This method is used to prepend one or more elements to the beginning of the array.
Q.39 What is the 'Strict Mode in JavaScript
Strict Mode adds certain compulsions to JavaScript. Under the strict Mode, JavaScript shows errors for a piece of code, which did not show an error before, but might be problematic and potentially unsafe. Strict Mode also solves some mistakes that hamper the JavaScript engines from working efficiently.
Q.40 How to enable strict mode
Strict mode can be enabled by adding the string literal "use strict" above the file.
Q.41 What is the way to get the status of a CheckBox
The status can be acquired as follows - alert(document.getElementById('checkbox1').checked); If the CheckBox is checked, this alert will return TRUE.
Q.42 What is a window.onload and onDocumentReady
The onload function is not run until all the information on the page is loaded. This leads to a substantial delay before any code is executed. onDocumentReady loads the code just after the DOM is loaded. This allows early manipulation of the code.
Q.43 How closures work in JavaScript
The closure is a locally declared variable related to a function that stays in memory when it has returned.
Q.44 How can a value be appended to an array
A value can be appended to an array in the given manner - arr[arr.length] = value;
Q.45 What are the important properties of an anonymous function in JavaScript
A function that is declared without any named identifier is known as an anonymous function. In general, an anonymous function is inaccessible after its declaration.
Q.46 What is the difference between .call() and .apply()
The function .call() and .apply() are very similar in their usage except a little difference. .call() is used when the number of the function's arguments are known to the programmer, as they have to be mentioned as arguments in the call statement. On the other hand, .apply() is used when the number is not known. The function .apply() expects the argument to be an array. The basic difference between .call() and .apply() is in the way arguments are passed to the function.
Q.47 What is event bubbling
JavaScript allows DOM elements to be nested inside each other. In such a case, if the handler of the child is clicked, the handler of the parent will also work as if it were clicked too.
Q.48 How are object properties assigned
Assigning properties to objects is done in the same way as a value is assigned to a variable. For example, a form object's action value is assigned as 'submit' in the following manner - Document. form.action="submit"
Q.49 What is the role of deferred scripts in JavaScript
The HTML code's parsing during page loading is paused by default until the script has not stopped executing. If the server is slow or the script is particularly heavy, then the web page is delayed. While using Deferred, scripts delays execution of the script till the time the HTML parser is running. This reduces the loading time of web pages, and they get displayed faster.
Q.50 What does the following statement declare? var myArray = [[[]]];
It declares a three-dimensional array.
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