In JavaScript, var, let, and const are used for declaring variables, but they differ in scope and mutability: var is function-scoped and can be re-declared and updated, while let and const are block-scoped, with let allowing updates and const preventing both updates and re-declarations.

forEach() will not return anything. forEach() returns undefined. filter() method will return an array of matching elements else will return an empty array if no matching happens. If you have a requirement to modify the current array and are expecting a modified one, then you should go with map() .

Arrow functions, introduced in ES6, offer a concise syntax for writing functions in JavaScript, often used for their readability and lexical scoping, while regular functions provide more flexibility and dynamic behavior .

In JavaScript, a Promise is an object that will produce a single value some time in the future. If the promise is successful, it will produce a resolved value, but if something goes wrong then it will produce a reason why the promise failed. The possible outcomes here are similar to that of promises in real life.

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives a function access to its outer scope. In JavaScript, closures are created every time a function is created, at function creation time.