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JavaScript Interview Questions and Answers
Question 1:
What is JavaScript?
JavaScript is a high-level, interpreted programming language that is primarily used for adding interactivity to web pages. It was initially created to run in web browsers but has now expanded its usage to other environments such as server-side development and mobile application development. JavaScript provides a way to manipulate web page elements, handle events, interact with servers, and perform various other tasks.
Question 2:
What are the data types in JavaScript?
JavaScript has several built-in data types, including:
- Number: represents numeric values
- String: represents textual data
- Boolean: represents true or false values
- Object: represents a collection of key-value pairs
- Array: represents an ordered list of values
- Null: represents the absence of any object value
- Undefined: represents an uninitialized variable
Question 3:
What is the difference between == and === in JavaScript?
In JavaScript, == (equality operator) compares two values for equality, performing type coercion if necessary. On the other hand, === (strict equality operator) compares two values for equality without performing type coercion. It checks for both value equality and type equality. For example, 5 == '5' would return true, while 5 === '5' would return false.
Question 4:
What are closures in JavaScript?
A closure is a function that has access to its own scope, the scope in which it is defined, and the scope of its outer function. It remembers the values in its lexical scope even when the outer function has finished executing. Closures are often used to create private variables and encapsulate functionality. They can be created by defining a function inside another function.
Question 5:
What is the difference between let, const, and var in JavaScript?
- var: Variables declared with var are function-scoped. They can be redeclared and reassigned within their scope and are hoisted to the top of their scope.
- let: Variables declared with let are block-scoped. They can be reassigned within their scope but cannot be redeclared in the same scope. They are not hoisted.
- const: Variables declared with const are also block-scoped but cannot be reassigned or redeclared. They are used for values that are intended to remain constant throughout the program.
Question 6:
What is the event delegation in JavaScript?
Event delegation is a technique in JavaScript where instead of attaching an event handler to each individual element, a single event handler is attached to a parent element. The event is then handled when it bubbles up to the parent, and the target element that triggered the event can be identified using the event object's properties. This approach is useful when dealing with dynamic content or large sets of elements, as it reduces the number of event handlers needed.
Question 7:
What is the difference between synchronous and asynchronous JavaScript?
Synchronous JavaScript executes tasks in a sequential manner, where each task waits for the previous one to complete before executing. Asynchronous JavaScript, on the other hand, allows tasks to be executed concurrently without waiting for the completion of previous tasks. Asynchronous operations, such as fetching data from a server or handling user interactions, are typically implemented using callbacks, promises, or async/await syntax in JavaScript.
Question 8:
What is the purpose of the 'this' keyword in JavaScript?
The 'this' keyword in JavaScript refers to the object that is currently executing the code. It allows you to access properties and methods within the current object's scope. The value of 'this' can vary depending on how a function is called. In a method, 'this' refers to the object that the method belongs to. In a standalone function, 'this' refers to the global object (e.g., the window object in a browser) or may be undefined in strict mode.
Question 9:
What is event bubbling and event capturing in JavaScript?
Event bubbling and event capturing are two mechanisms in JavaScript for handling events that occur on nested elements. Event bubbling refers to the propagation of an event from the innermost element up to the outermost element. In contrast, event capturing involves capturing the event at the outermost element first and then propagating it down to the innermost element. By default, event bubbling is used in JavaScript, but event capturing can be enabled by setting the 'capture' parameter to true in the event listener.
Question 10:
What are arrow functions in JavaScript?
Arrow functions are a shorthand syntax for writing function expressions in JavaScript. They provide a more concise syntax compared to traditional function expressions and automatically bind 'this' lexically. Arrow functions are always anonymous and cannot be used as constructors. They are commonly used in scenarios where a shorter syntax is preferred for defining functions, especially for one-liners or when working with callbacks.
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