JavaScript Interview Questions and Answers for 5 years of experience — 2023
Unlock your full potential in your next JavaScript interview with this in-depth collection of questions and answers, tailored for experienced developers with 5 years of experience. Brush up on advanced concepts such as closures, event loops, and transpilers to impress potential employers and secure your spot as a top candidate.
Gain a competitive edge in your next JavaScript interview with this comprehensive list of questions and answers, tailored for candidates with 5 years of experience. Stay current with the latest developments in the field with knowledge up to 2023. Master advanced concepts like closures, event loops, and transpilers to impress potential employers and land your dream job.
Can you explain the difference between let and var in JavaScript?
Answer: The main difference between “let” and “var” is that variables declared with “let” have block scope, while those declared with “var” have function scope.
Can you explain closure in JavaScript?
Answer: A closure is a function that has access to variables in its parent scope, even after the parent function has returned. Closures can be used to maintain state, or create private variables and methods.
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How do you handle errors in JavaScript?
Answer: Errors in JavaScript can be handled using try-catch statements. The code that might throw an error is placed in the try block, and the error handling code is placed in the catch block.
Can you explain the difference between == and === in JavaScript?
Answer: “==” compares values for equality and performs type coercion if necessary. “===” compares values for equality without performing type coercion. It is considered best practice to use “===” in most cases.
Can you explain the event loop in JavaScript?
Answer: The event loop is a mechanism that allows JavaScript to perform non-blocking I/O operations. It works by continually checking the message queue for messages (events) and processing them in the order they were received. This allows the main thread to continue executing while waiting for other events to occur.
Can you explain the difference between synchronous and asynchronous code in JavaScript?
Answer: Synchronous code is executed in the order it is written, while asynchronous code is executed at a later time, not blocking the execution of other code.
Can you explain the difference between call() and apply() in JavaScript?
Answer: The call() method allows you to call a function and set the this value inside the function, and arguments passed as individual arguments. The apply() method allows you to call a function and set the this value inside the function, and arguments passed as an array.
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Can you explain event bubbling and event capturing in JavaScript?
Answer: Event bubbling is a way of event propagation in which an event propagates from the innermost element outwards. Event capturing is the opposite, in which an event propagates from the outermost element inwards.
Can you explain the difference between a forEach loop and a map loop in JavaScript?
Answer: A forEach loop is used to iterate over an array and perform a function on each element, but it does not return a new array. A map loop is used to iterate over an array, perform a function on each element, and return a new array with the results.
How can you optimize the performance of a JavaScript application?
Answer: Some ways to optimize the performance of a JavaScript application include: minimizing the use of global variables, minimizing the use of complex CSS selectors, reducing the number of DOM elements, and minifying and concatenating JavaScript files.
Can you explain the difference between a Promise and a callback in JavaScript?
Answer: A callback is a function passed as an argument to another function, which is then invoked inside that function. A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value. Promises provide a more powerful and flexible way to handle asynchronous operations compared to callbacks.
Can you explain the difference between a class and a constructor in JavaScript?
Answer: A class is a blueprint for creating objects (a particular data structure), providing initial values for state (member variables or properties) and implementations of behavior (member functions or methods). A constructor is a special type of function that is used to initialize an object created with a class. It is automatically called whenever a new instance of the class is created.
Can you explain how prototypal inheritance works in JavaScript?
Answer: In JavaScript, objects can inherit properties and methods from other objects through a mechanism called prototypal inheritance. Each object has a private property called “prototype” that holds a reference to another object. When trying to access a property or method of an object, if it does not exist on the object itself, JavaScript will look for it in the object’s prototype, and so on up the prototype chain.
Can you explain the difference between == and Object.is() in JavaScript?
Answer: Both “==” and Object.is() are used to compare two values for equality. However, “==” has some quirks such as type coercion and special handling of NaN and -0. On the other hand, Object.is() is a more robust and precise comparison, it does not do type coercion and it can distinguish between -0 and +0, NaN and NaN.
Can you explain the difference between a Set and a Map in JavaScript?
Answer: A Set is a collection of unique values. A Map is a collection of key-value pairs, where both the key and the value can be of any type. Sets are useful when you need to keep track of unique values, while Maps are useful when you need to associate a value with a specific key.
Can you explain the difference between a for loop and a for-of loop in JavaScript?
Answer: A for loop is used to iterate over an array, by using the index of each element. A for-of loop is used to iterate over the values of an array, ignoring the index. for-of loop is introduced in es6, it makes the code more readable and less error-prone.
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Can you explain the difference between a for-in loop and a for-of loop in JavaScript?
Answer: A for-in loop is used to iterate over the properties of an object, while a for-of loop is used to iterate over the values of an array or an iterable object. for-in loop iterates over the property names of an object, for-of loop iterates over the values of an object.
Can you explain the difference between Array.reduce() and Array.filter() in JavaScript?
Answer: Array.reduce() is used to apply a function to each element in an array and reduce the array to a single value. Array.filter() is used to filter the elements of an array based on a provided condition and returns a new array with the elements that pass the condition.
Can you explain the difference between a async function and a generator function in JavaScript?
Answer: An async function is a function that returns a promise and can be awaited. A generator function is a special type of function that can be paused and resumed multiple times. It allows you to write asynchronous code that looks synchronous. They are typically used to iterate over large data sets, handle infinite sequences, or perform long-running tasks like polling an API.
Can you explain what is hoisting in JavaScript?
Answer: Hoisting is a behavior in JavaScript where variable and function declarations are moved to the top of their scope. This means that a variable or function can be used before it is declared in the code. However, only the declaration is hoisted, not the assignment.
Can you explain what is Currying in JavaScript?
Answer: Currying is a technique in functional programming where a function is transformed into a sequence of functions, each taking a single argument. Currying allows for easier composition and reuse of functions, as well as making it easier to understand and debug code.
Can you explain what is a Higher-Order function in JavaScript?
Answer: A Higher-Order function is a function that takes one or more functions as arguments and/or returns a function as its result. Higher-order functions are a key concept in functional programming, and are often used to create more expressive and reusable code.
Can you explain what is a Pure function in JavaScript?
Answer: A Pure function is a function that always returns the same output for the same input, and does not have any side-effects. In other words, a pure function only depends on its input and does not depend on any external state. Pure functions are easier to test, understand and reason about, and they are considered best practice in functional programming.
Can you explain what is a Decorator in JavaScript?
Answer: A Decorator is a design pattern that allows behavior to be added to an individual object, either statically or dynamically, without affecting the behavior of other objects from the same class. In JavaScript, decorators are a way to add metadata to a class or its members, like adding an annotation in other programming languages.
Can you explain what is a Transpiler in JavaScript?
Answer: A Transpiler is a source-to-source compiler that translates source code written in one programming language into another language. In JavaScript, transpilers are often used to convert modern JavaScript code (ES6, ES7, etc) into older versions of JavaScript that are compatible with older browsers. Some popular transpilers include Babel, TypeScript and Traceur.
Can you explain what is a WeakMap in JavaScript?
Answer: A WeakMap is a collection of key-value pairs, where the keys must be objects and the values can be of any type. The main difference between a WeakMap and a Map is that the references to the keys in a WeakMap are weak, meaning that they are not held strong by the WeakMap and can be garbage-collected if they are not used elsewhere. This makes WeakMaps suitable for use cases where you want to associate data with an object without preventing that object from being garbage-collected.
Can you explain what is a Proxy in JavaScript?
Answer: A Proxy is an object that acts as an intermediary between the code that accesses an object and the object itself. Proxies allow you to intercept and manipulate operations on the target object, such as property access, assignment, method calls, and more. They are useful for creating objects that have custom behavior, such as logging, access control, or performance monitoring.
Can you explain what is a Reflect in JavaScript?
Answer: Reflect is a built-in object in JavaScript that provides methods for intercepting and manipulating operations on other objects. Reflect has the same methods as the Proxy object but it does not allow you to change the behavior. Reflect is mainly used to provide a consistent way of interacting with objects, regardless of whether they are implemented using the object literal notation or a class.
Can you explain what is a Template literals in JavaScript?
Answer: Template literals are a new feature introduced in ECMAScript 6, which allows you to create strings with embedded expressions. They are enclosed in backticks (`) rather than single or double quotes and allow for multiline strings and string interpolation using the ${expression} syntax.
Can you explain what is a Spread operator in JavaScript?
Answer: The spread operator is a new feature introduced in ECMAScript 6, which allows you to expand an iterable (an array or a string) into individual elements. The spread operator is denoted by three dots (…). It can be used in various ways to copy an array, concatenate arrays, and pass elements of an array as function arguments. It is also used to spread an object’s properties into a new object.
Can you explain what is a Ternary operator in JavaScript?
Answer: The ternary operator is a shorthand way of writing an if-else statement. It uses the “?” and “:” symbols to return one value if a condition is true and another value if the condition is false. The syntax for the ternary operator is: condition ? valueIfTrue : valueIfFalse.
Can you explain what is a Destructuring assignment in JavaScript?
Answer: Destructuring assignment is a feature introduced in ECMAScript 6, which allows you to extract values from arrays or objects and assign them to variables in a concise and readable way. It helps to unpack values from arrays or properties from objects, into distinct variables.
Can you explain what is a Rest parameter in JavaScript?
Answer: Rest parameter is a feature introduced in ECMAScript 6, which allows you to represent an indefinite number of arguments as an array. The rest parameter is denoted by three dots (…) before the parameter name. It allows a function to accept any number of arguments, regardless of how many are defined in the function signature.
Can you explain what is a Default parameter in JavaScript?
Answer: Default parameter is a feature introduced in ECMAScript 6, which allows you to specify a default value for a function parameter if no value is provided. This means that if the function is called with fewer arguments than expected, the missing parameters will automatically be set to the default values.
Can you explain what is a Generators in JavaScript?
Answer: A generator is a special type of function that can be paused and resumed multiple times. Generators are defined using the “function*” syntax and can be used to generate a sequence of values, handle infinite sequences, or perform long-running tasks like polling an API. Generators use the “yield” keyword to produce a value and the “next()” method to resume execution.
Can you explain what is a Async/Await in JavaScript?
Answer: Async/Await is a feature introduced in ECMAScript 2017, which allows you to write asynchronous code that looks synchronous. Async/await is built on top of Promises and makes it easier to handle asynchronous operations. The “async” keyword is used to define an asynchronous function, and the “await” keyword is used to wait for a promise to resolve before continuing execution.
Can you explain what is a Modules in JavaScript?
Answer: A module is a piece of code that is executed once it is loaded. JavaScript has a built-in module system called CommonJS, which is supported by Node.js. Modules provide a way to organize code and reuse it across different files and projects. They also provide a way to encapsulate variables and functions, making them private to the module.
Can you explain what is a Webpack in JavaScript?
Answer: Webpack is a popular JavaScript module bundler that can be used to bundle and optimize JavaScript, CSS, and other assets for a web application. It allows developers to write code in modular fashion and then bundle all of the modules together into a single file (or a few files) that can be loaded by the browser. It also provides many other functionalities like code splitting, minification, and more.
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