Top 37 javascript interview questions and answers
Top 37 javascript interview questions and answers
1. What is JavaScript?
Answer: JavaScript is a high-level, interpreted programming language commonly used to create interactive effects within web browsers. It’s a key technology of the World Wide Web alongside HTML and CSS.
2. What are the data types in JavaScript?
Answer: JavaScript data types include:
- Primitive types:
String,Number,Boolean,Null,Undefined,Symbol,BigInt - Non-primitive types:
Object
3. What is the difference between let, const, and var?
Answer:
varis function-scoped and can be re-declared.letis block-scoped and cannot be re-declared but can be updated.constis block-scoped, cannot be re-declared, and must be initialized during declaration. It also cannot be updated.
4. What are JavaScript closures?
Answer: A closure is a function that has access to its own scope, the scope of the outer function, and the global scope. It allows a function to access variables from an enclosing scope or environment even after it leaves the scope in which it was declared.
5. Explain the difference between == and ===.
Answer:
==is the abstract equality operator that compares two values for equality after converting both values to a common type.===is the strict equality operator that compares two values for equality without performing type conversion. Both the value and the type must be the same.
6. What is a promise in JavaScript?
Answer: A promise is an object representing the eventual completion or failure of an asynchronous operation. Promises have three states: pending, fulfilled, and rejected.
7. What is an arrow function?
Answer: An arrow function is a shorter syntax for writing functions in ES6. It does not have its own this context and is always anonymous.
const add = (a, b) => a + b;
8. What is the difference between null and undefined?
Answer:
undefinedmeans a variable has been declared but not yet assigned a value.nullis an assignment value that represents no value or no object.
9. What are template literals?
Answer: Template literals are string literals allowing embedded expressions. You can use multi-line strings and string interpolation features with them.
let name = "John";
let greeting = `Hello, ${name}!`;
10. What is an Immediately Invoked Function Expression (IIFE)?
Answer: An IIFE is a function that is executed immediately after it is defined. It helps to create a local scope and avoid polluting the global namespace.
(function() {
console.log("This is an IIFE!");
})();
11. What is event delegation?
Answer: Event delegation is a technique involving adding a single event listener to a parent element to manage events from its child elements. It utilizes event bubbling to handle events at a higher level in the DOM tree.
12. Explain the concept of hoisting in JavaScript.
Answer: Hoisting is JavaScript’s default behavior of moving declarations (variables and functions) to the top of their containing scope during compilation. Only the declarations are hoisted, not the initializations.
13. What are JavaScript modules?
Answer: JavaScript modules are reusable pieces of code that can be exported from one module and imported into another. ES6 introduced the import and export syntax to handle modules.
14. What is the this keyword in JavaScript?
Answer: The this keyword refers to the object that is executing the current function. Its value depends on the context in which it is used (global, object, function, etc.).
15. What is a callback function?
Answer: A callback function is a function passed into another function as an argument and is executed after some operation has been completed.
16. What is the purpose of the Array.prototype.map() method?
Answer: The map() method creates a new array populated with the results of calling a provided function on every element in the calling array.
17. How does the async and await syntax work?
Answer: async functions return a promise, and await pauses the execution of the function until the promise is resolved or rejected. It allows writing asynchronous code in a synchronous-like manner.
18. What is the difference between call() and apply()?
Answer:
call()invokes a function with a giventhisvalue and arguments provided individually.apply()invokes a function with a giventhisvalue and arguments provided as an array.
19. What are higher-order functions?
Answer: Higher-order functions are functions that can take other functions as arguments or return functions as their result.
20. What is the event loop in JavaScript?
Answer: The event loop is a mechanism that handles the execution of multiple chunks of your program over time. It allows JavaScript to perform non-blocking operations, despite being single-threaded.
21. What are JavaScript prototypes?
Answer: Prototypes are the mechanism by which JavaScript objects inherit features from one another. Every object in JavaScript has a prototype property, which is a reference to another object from which it can inherit properties and methods.
22. What is the purpose of Object.create()?
Answer: Object.create() creates a new object with the specified prototype object and properties.
23. What are default parameters in JavaScript?
Answer: Default parameters allow you to initialize named parameters with default values if no value or undefined is passed.
function greet(name = "Guest") {
return `Hello, ${name}!`;
}
24. What is the difference between synchronous and asynchronous code?
Answer:
- Synchronous code is executed sequentially, one line at a time.
- Asynchronous code allows the program to continue running while waiting for an operation to complete, typically using callbacks, promises, or async/await.
25. Explain the reduce() method in JavaScript.
Answer: The reduce() method executes a reducer function on each element of the array, resulting in a single output value.
const sum = [1, 2, 3, 4].reduce((acc, curr) => acc + curr, 0); // 10
26. What is NaN in JavaScript?
Answer: NaN stands for “Not-a-Number” and is a value representing an undefined or unrepresentable value in arithmetic operations.
27. What are JavaScript events?
Answer: Events are actions or occurrences that happen in the system you are programming, which the system tells you about so you can respond to them in some way.
28. What is event bubbling and event capturing?
Answer: Event bubbling is when an event starts at the most specific element and then flows upward to the least specific ones (document). Event capturing is the opposite, where the event starts at the least specific element and flows downward to the most specific one.
29. What is a Promise.all()?
Answer: Promise.all() takes an array of promises and returns a single promise that resolves when all the promises in the array have resolved, or rejects if any of the promises rejects.
30. What is the difference between an undefined and a null value in JavaScript?
Answer: undefined is a variable that has been declared but not yet assigned a value, whereas null is an assignment value representing no value.
31. What is the difference between the forEach() and map() methods in JavaScript?
Answer:
forEach()executes a provided function once for each array element but does not return anything.map()creates a new array with the results of calling a provided function on every element in the calling array.
32. What is the purpose of Array.prototype.filter()?
Answer: The filter() method creates a new array with all elements that pass the test implemented by the provided function.
33. What is a spread operator and how is it used?
Answer: The spread operator (...) allows an iterable (like an array or string) to be expanded in places where zero or more arguments or elements are expected.
const arr = [1, 2, 3];
const newArr = [...arr, 4, 5];
34. What are template literals in JavaScript?
Answer: Template literals are string literals allowing embedded expressions. They are enclosed by backticks (`) instead of single or double quotes.
35. What is the difference between Function Declaration and Function Expression?
Answer:
- Function Declaration: A function, declared as a separate statement in the main code flow.javascript
function sum(a, b) {
return a + b;
}
- Function Expression: A function, created inside an expression or inside another syntax construct. It can be anonymous.javascript
const sum = function(a, b) {
return a + b;
};
36. Explain the concept of Debouncing in JavaScript.
Answer: Debouncing is a programming practice used to ensure that time-consuming tasks do not fire so often, preventing performance issues. It ensures that a function is executed only once after a specified period of time has elapsed since the last time it was invoked.
function debounce(func, wait) {
let timeout;
return function(...args) {
clearTimeout(timeout);
timeout = setTimeout(() => func.apply(this, args), wait);
};
}
37. What is the purpose of the Array.prototype.some() method?
Answer: The some() method tests whether at least one element in the array passes the test implemented by the provided function. It returns a Boolean value.
const array = [1, 2, 3, 4, 5];
const hasEvenNumber = array.some(num => num % 2 === 0); // trueFor More Interview Tips and Questions : Click Here
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