JavaScript Subtopics for Exam Preparation

JavaScript (JS) is a lightweight, interpreted programming language used for creating interactive and dynamic content on websites. It runs in web browsers (client-side) and on servers using Node.js. JavaScript enables features like form validation, animations, dropdown menus, and real-time updates. Along with HTML and CSS, JavaScript is one of the three core technologies of the web.

var has function scope and is hoisted, can be redeclared and reassigned. let has block scope, is not hoisted, can be reassigned but not redeclared. const has block scope, must be initialized at declaration, and cannot be reassigned. Modern JavaScript recommends using const by default and let when reassignment is needed. var usage is generally discouraged in modern code.

A closure is a function that has access to its outer scope's variables even after the outer function has finished executing. Closures are used for: data privacy (emulating private variables), function factories, currying, and maintaining state in async operations. Closures are a fundamental JavaScript concept asked in interviews and are created every time a function is created.

== (loose equality) compares values after type coercion. For example: '5' == 5 returns true. === (strict equality) compares both value AND type without coercion. '5' === 5 returns false because one is string and one is number. Always use === for comparisons to avoid unexpected behavior. This is one of the most common JavaScript interview questions.

async/await is a modern JavaScript syntax (ES2017) for handling Promises in a readable, synchronous-looking way. The async keyword makes a function return a Promise. The await keyword pauses execution inside an async function until the Promise resolves. It simplifies error handling using try/catch blocks. async/await is the preferred way to handle asynchronous operations in modern JavaScript.