var: when we need function-scoped variables.
let: when we need block-scoped variables that can be reassigned.
const: when we need block-scoped variables with values that should not be reassigned
Undefined: A situation when a variable that has been declared but hasn't been assigned a value yet.
Use case of undefined:
1.When a variable is declared but not initialized.
2.Default value for function parameters.
3.When object property doesn't exist.
Null: Represent the absence of a value or the emptiness of an object.
Use case of null:
1.To indicate that a variable or object has no value.
2.Way to clear or reset a variable or object.
REST API means - Representational State Transfer Application Programming Interface. A REST API provides a standardized way for software applications to request and exchange data over the internet, makes easier for developers to build and integrate different services and systems.