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
}
