Most Asked Javascript Interview Questions
Top 10 Most Commonly Asked JavaScript Interview Questions
Some of the most commonly asked JavaScript interview questions include topics such as data types, functions, scope, closures, prototypes, and object-oriented programming concepts. Candidates are also often asked about their knowledge of vanilla JavaScript versus popular frameworks, debugging techniques, and handling errors. Additionally, employers may ask about the candidate's understanding of the DOM, AJAX, and various APIs, as well as their experience with responsive design and cross-browser compatibility. Strong problem-solving skills and the ability to write clean and efficient code are also important qualities that are often tested during JavaScript interviews.
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1 - What is Javascript and what are its main features?
Javascript is a high level, interpreted programming language primarily used for creating interactive and dynamic web content. It is also used for server side development and mobile application development. Its main features include its lightweight, cross platform compatibility, and the ability to manipulate webpage elements.
2) What is the difference between ‘var’, ‘let’, and ‘const’ in Javascript?
‘var’ and ‘let’ are used for variable declarations in Javascript, with ‘var’ being function scoped and ‘let’ being block scoped. However, while ‘var’ allows the variable to be reassigned, ‘let’ does not. On the other hand, ‘const’ is used to declare a constant variable that cannot be reassigned or redeclared.
3) What is a closure in Javascript?
A closure is a combination of a function and the lexical environment in which it wased. This allows the function to access variables from its outer scope even after that scope has been executed. It is commonly used to create private variables or for encapsulating data.
4) What are the different types of errors in Javascript?
There are three main types of errors in Javascript: syntax errors, runtime errors, and logical errors. Syntax errors occur when the code is not valid and cannot be interpreted by the browser. Runtime errors occur during the execution of the code, while logical errors do not cause the code to fail but produce unexpected results.
5) How does prototypal inheritance work in Javascript?
In Javascript, objects can inherit properties and methods from other objects. This is known as prototypal inheritance. When an object is created, it contains a hidden property called proto, which points to its prototype. If a property or method is not found in the object, it is then looked for in its prototype, and so on until it reaches the top of the prototype chain.
6) Explain the concept of event bubbling in Javascript.
Event bubbling is a mechanism in which an event propagates from the innermost element outwards to the outermost element. For example, if a button is clicked inside a div element, the click event will first be triggered on the button, then the div, and then any other parent elements that may be present.
7) What is a higher order function in Javascript?
A higher order function is a function that takes another function as an argument or returns a function as its result. This allows for functional programming concepts, such as passing functions as callbacks or creating new functions from existing functions.
8) How can you improve Javascript performance?
Javascript performance can be improved by using efficient coding techniques, such as minimizing global variables, avoiding unnecessary DOM manipulations, and using asynchronous code whenever possible. Additionally, using tools like minification and code bundlers can improve overall page performance.
9) What is the difference between ‘== and ’===' operators in Javascript?
The ‘==’ operator checks for equality in value, while the ‘===’ operator checks for equality in both value and data type. This means that ‘==’ may return true for two values that are actually of different data types, but ‘===’ will only return true if the values are of the same data type.
10) How can you handle errors in Javascript?
Errors in Javascript can be handled using ‘try catch’ blocks, which allow you to catch and handle specific types of errors in your code. Additionally, you can use the ‘finally’ block to execute code regardless of whether an error was caught or not.
11) What is a promise in Javascript?
A promise is an object that represents the eventual completion or failure of an asynchronous operation. It allows for better handling of asynchronous code, avoiding callback hell and enabling a more readable and sequential flow of code.
12) How does event delegation work in Javascript?
Event delegation is a technique where instead of attaching an event listener to each individual element, it is attached to a parent element that contains multiple child elements. This allows for the handling of events on dynamically added elements and helps in improving performance.
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13) What is the difference between ‘null’ and ‘undefined’ in Javascript?
Both ‘null’ and ‘undefined’ represent the absence of a value. However, ‘null’ is explicitly defined by the programmer, while ‘undefined’ means that a variable has been declared but not assigned a value. Additionally, ‘undefined’ is also the default return value of functions that do not have a return statement.
14) What is the difference between ‘let’ and ‘const’ in terms of hoisting?
‘let’ variables are hoisted to the top of their block scope, but they are not initialized until their declaration is reached in the code. On the other hand, ‘const’ variables are also hoisted, but they must be initialized during their declaration, otherwise, an error will be thrown.
15) How do you implement inheritance in Javascript?
Inheritance can be implemented in Javascript using either prototypal inheritance or ES6 classes. In prototypal inheritance, the prototype chain is used to inherit properties and methods from parent objects. ES6 classes provide a syntax for creating and extending classes, making inheritance more similar to traditional OOP languages.
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