Variables declared with "var" are function-scoped. This means they are visible throughout the entire function where they aredeclared.If a variable is declared with "var" outside any function, it becomes a global variable, visible throughout the entire script.

The "let" keyword was introduced in ECMAScript 6 (ES6) and has block scope, which means the variable is only accessible within the block or statement where it is defined.

Similar to let, variables declared with "const" are working just in their block scope. The primary feature of "const" is that once a value is assigned, it cannot be reassigned.

"null" and "undefined" are two distinct primitive values in JavaScript.
let myValue = null
It can be assigned to variables or properties to indicate that they intentionally have no meaningful value.

Variables that have been declared but not assigned a value, automatically it gives the "undefined" as a output.
let myVariable; console.log(myVariable); // Outputs: undefined
It is also works when a parameter is missing in a funtion that already called. or When we trying to access an object property that doesn't exist, the result will be also "undefined".

REST API, which stands for Representational State Transfer Application Programming Interface, is a set of architectural principles and constraints used for designing networked applications.
RESTful APIs are widely used in web development for building scalable and interoperable services. They leverage the HTTP protocol for communication, making use of standard HTTP methods for CRUD (Create, Read, Update, Delete) operations on resources.