JavaScript Interview Questions and Answers

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Here are the best JavaScript interview questions and Answers to crack your interview easily.

What is Javascript?

Javascript is a scripting language(supports scripts) for Web pages but it is also used in non-browser environments as well. It is a powerful, lightweight, interpreted, scripting language with first-class functions (i.e. the language supports passing functions as arguments to other functions).

In order to add dynamic interactivity to web pages Javascript is embedded within Hypertext Markup Language (HTML). Since it runs on the client-side of the web so it can be used to design/program how the web pages behave on the occurrence of a particular event. That’s the reason why it is widely used for the behavior of web pages.

What are the variable naming conventions in JavaScript?

The following rules are to be followed while naming variables in JavaScript:

  1. You should not use any of the JavaScript reserved keyword as variable name. For example, break or boolean variable names are not valid.
  2. JavaScript variable names should not start with a numeral (0-9). They must begin with a letter or the underscore character. For example, 123name is an invalid variable name but _123name or name123 is a valid one.
  3. JavaScript variable names are case sensitive. For example, Test and test are two different variables.

How does TypeOf Operator work?

The typeof operator is used to get the data type of its operand. The operand can be either a literal or a data structure such as a variable, a function, or an object. It is a unary operator that is placed before its single operand, which can be of any type. Its value is a string indicating the data type of the operand.

When should I use Arrow Functions in ES6?

‘m now using the following rule of thumb for functions in ES6 and beyond:

  • Use function in the global scope and for Object.prototype properties.
  • Use class for object constructors.
  • Use => everywhere else.
javascript interview questions

Why use arrow functions almost everywhere?

  • Scope safety: When arrow functions are used consistently, everything is guaranteed to use the same thisObject as the root. If even a single standard function callback is mixed in with a bunch of arrow functions there’s a chance the scope will become messed up.
  • Compactness: Arrow functions are easier to read and write. (This may seem opinionated so I will give a few examples further on).
  • Clarity: When almost everything is an arrow function, any regular function immediately sticks out for defining the scope. A developer can always look up the next-higher function statement to see what the thisObject is.

What is the definition of global variables? In what way, these variables are declared?  

A global variable is a special kind of variable in JavaScript. This variable is easy to use and also available across the entire length of the JavaScript code. Mainly, the var keyword is used whether to declare a global or local variable.

Basic JavaScript Interview Questions and Answers

What is the definition of the prompt box?  

The Input provided by any user in the JavaScript is entered with the help of a prompt box. While putting forward the data or the input, the prompt box allows the user to do this with the help of a text box. To include the number, a label box is also used.

What is Callback?

A callback is a plain JavaScript function passed to some method as an argument or option. It is a function that is to be executed after another function has finished executing, hence the name ‘call back‘. In JavaScript, functions are objects. Because of this, functions can take functions as arguments, and can be returned by other functions.

List out different types of errors in JavaScript?

Different types of errors in JavaScript are:

  • Run time errors- errors occurred due to misrepresentation of HTML commands.
  • Load time errors- errors that occurred at the time of web page loading are known as load time errors. These errors may occur due to improper syntaxes
  • Logical errors- logical errors may occur due to improper logical performance of the function.

What are the terms BOM and DOM in JavaScript?

  • BOM- BOM is a Browser Object Model, in which the window object is supported by all the browsers. The JavaScript objects, variables, and functions also become members of the window object. This model deals with the objects of browsers like location, navigator, history, and screen.
  • DOM- DOM is a Document Object Model in JavaScript that helps to access document and HTML elements. When a web page is loaded the browser creates a DOM for the page. All the objects on the page all arranged in order.

What is the working 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.

The setTimeout(function, delay) function is used to start a timer that calls a particular function after the mentioned delay. The setInterval(function, delay) function repeatedly executes the given function in the mentioned delay and only halts when canceled. The clearInterval(id) function instructs the timer to stop.

Timers are operated within a single thread, and thus events might queue up, waiting to be executed.

What are 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.

What a pop()method in JavaScript is?

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.

Explain the role of closures in JavaScript.

Ans. In JavaScript, the script creates a closure at the time of function creation. It is a local variable that stays in the memory even after the function completes its execution. It has access to the variables in three scopes: variable in its own scope, variables in the enclosing function’s scope, and global variables. Closures are crucial in JavaScript to ensure the privacy of variables, that is, to create private variables. Since JavaScript has no access modifiers, closures allow the programmer to create variables that are not directly accessible.

Is it possible to remove the content from a website? If yes, how?

Yes, there are a few possible ways to remove content from the website at will.

The easiest way is to hide the content from the browser you want to remove. The display property of the JavaScript style attribute helps us achieve the task. Whenever we add any content to the webpage, its default display sets to block (display: block). We can change the value to none (display: none). This will prevent the content from showing in the browser window. This approach only hides the details, it will still be available in the source code, and you can access it from the Elements window.

Another way to do so is by removing the element node corresponding to the content. This technique completely removes the data from the webpage as well as the code. We can do this by .removeChild() method. Or if we are dealing with an array (including a NodeList), then we can easily empty the array, and get the same result.

The decision of selecting the appropriate method for the situation depends on the programmer and his/ her needs.

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