Variables in JavaScript
A VARIABLE IS A " NAMED STORAGE " FOR DATA .
Declaration :- In JavaScript variables can be declared using
let
const &
var
let and const were introduced in 2015 ( ES6 )
var :
it is function ( if declared inside a function ) or Global scoped ( if declared globally ) . it allows variable to be both Re-assigned and re-defined . Allows Hoisting .
scope : Function . Re-assignable : Yes . Mutable : Yes . Temporal dead zone : No .
let :
Block scoped . Allows Reassignment .
scope : Block . Re-assignable : Yes . Mutable : Yes . Temporal dead zone : Yes .
const :
used to declare constant . Once a value is assigned to a constant using const , it cannot be changed or reassigned throughout the execution of the script . it is important to note that if the value is in the form of array or object , its elements or properties can still be modified .
scope : Block . Re-assignable : No . Mutable : Yes . Temporal dead zone : Yes .
Hoisting :
var declarations , wherever they occur in a script , are processed before any code within the script is executed . Declaring a variable anywhere in the code is equivalent to declaring it at the top ( in its block ) . this also means that a variable can appear to be used before it is declared . this behaviour is called HOISTING .
[ if we try to access var declared variable before their declaration we get undefined , not an error . But in case of let and const , if we try to access them before their declaration we get a reference error , this is known as Temporal dead zone . ]
Scope :
The context in which values and expressions are 'visible' or can be referenced . if a variable or other expression is not 'in the current scope' then it is unavailable for use .
Variable mutation :
A variable is said to have been mutated when its initial value has been changed afterward . A variable is said to be immutable if it can't be mutated .
Summary : -
| Feature | var | let | const |
| Scope | Function scope or global scope | Block scope | Block scope |
| Hoisting | Hoisted and initialized as undefined | Hoisted but not initialized (TDZ) | Hoisted but not initialized (TDZ) |
| Re-declaration | Allowed in the same scope | Not allowed in the same scope | Not allowed in the same scope |
| Re-assignment | Allowed | Allowed | Not allowed |
| Initialization | Can be declared without initialization | Can be declared without initialization | Must be initialized at declaration |
| Temporal Dead Zone (TDZ) | No | Yes | Yes |
