Const in JavaScript

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Tue Jun 06 2023 16:40:14 GMT+0000 (Coordinated Universal Time)

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const PI = 3.141592653589793;
PI = 3.14;      // This will give an error
PI = PI + 10;   // This will also give an error

// JavaScript const variables must be assigned a value when 
//   they are declared : 

// Correct
const PI = 3.14159265359;

// Incorrect
const PI;
PI = 3.14159265359;

// Always declare a variable with const when you know that 
// the value should not be changed.
// Use const when you declare:
// * A new Array
// * A new Object
// * A new Function
// * A new RegExp

//You can change the elements of a constant array :
// You can create a constant array :
const cars = ["Saab", "Volvo", "BMW"];
// You can change an element :
cars[0] = "Toyota";
// You can add an element :
cars.push("Audi");

// But you can NOT reassign the array :
const cars = ["Saab", "Volvo", "BMW"];
cars = ["Toyota", "Volvo", "Audi"]; 
// This creates an ERROR

// You can create a const object :
const car = {type:"Fiat", model:"500", color:"white"};
// You can change a property:
car.color = "red";
// You can add a property:
car.owner = "Johnson";

// But you can NOT reassign the object :
const car = {type:"Fiat", model:"500", color:"white"};
car = {type:"Volvo", model:"EX60", color:"red"};   
// ERROR

// Declaring a variable with const is similar to let when it
// comes to Block Scope. The x declared in the block, in this
// example, is not the same as the x declared outside the
// block:
const x = 10;
// Here x is 10
{
const x = 2;
// Here x is 2
}
// Here x is 10

// Declaring a variable with const is similar to let when it 
// comes to Block Scope. The x declared in the block, in this
// example, is not the same as the x declared outside the
// block:
var x = 2;     // Allowed
var x = 3;     // Allowed
x = 4;         // Allowed

// New Example
var x = 2;     // Allowed
const x = 2;   // Not allowed

{
let x = 2;     // Allowed
const x = 2;   // Not allowed
}

{
const x = 2;   // Allowed
const x = 2;   // Not allowed

  //New Example
const x = 2;     // Allowed
x = 2;           // Not allowed
var x = 2;       // Not allowed
let x = 2;       // Not allowed
const x = 2;     // Not allowed

{
  const x = 2;   // Allowed
  x = 2;         // Not allowed
  var x = 2;     // Not allowed
  let x = 2;     // Not allowed
  const x = 2;   // Not allowed

//New Example
const x = 2;       // Allowed
{
  const x = 3;   // Allowed
}
{
  const x = 4;   // Allowed
}
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https://www.w3schools.com/Js/js_const.asp