JavaScript Const
The const keyword in JavaScript is used to define variables that cannot be changed once they’re assigned a value. This prevents any modifications to the variable’s value.
Additionally, const doesn’t allow redeclaration of the same variable within the same block, and it provides block scope. It was introduced in ES2015 (ES6) for creating immutable variables.
Syntax:
const const_name;
const x;
Properties:
- Cannot be reassigned.
- It has Block Scope
- It can be assigned to the variable on the declaration line.
- It’s a Primitive value.
- The property of a const object can be changed but it cannot be changed to a reference to the new object
- The values inside the const array can be changed, it can add new items to const arrays but it cannot reference a new array.
- Re-declaring of a const variable inside different block scopes is allowed.
- Cannot be Hoisted.
- Creates only read-only references to value.
Examples of JavaScript Const
Cannot be Reassigned
Example 1: It describes that the const variable cannot be reassigned.
const x = 12;
x = 13;
x += 1;
Output:
Uncaught TypeError: Assignment to constant variable.
Example 2: It describes the const variable which contains the Block Scope.
const x = 22;
{
const x = 90;
console.log(x);
{
const x = 77;
console.log(x);
}
{
const x = 45;
console.log(x);
}
}
console.log(x);
Output
90 77 45 22
Variables must be Assigned
Example: It describes the const variable and assigned it after declaration.
const x;
x = 12;
Output:
Uncaught SyntaxError: Missing initializer in const declaration
Cannot be Hoisted
Example: It describes the const variable cannot be Hoisted.
x = 3;
console.log(x);
const x;
Output:
Uncaught SyntaxError: Missing initializer in const declaration
Const in Arrays
Example: It describes that the array values can be modified only reference to the array cannot be changed.
// Changing the content of array is
// possible in cost array
const arr1 = ["pankaj", "sumit", "chandan", "ajay"];
console.log(arr1.toString());
arr1[2] = "Narayan"; // possible
console.log(arr1.toString());
Output
pankaj,sumit,chandan,ajay pankaj,sumit,Narayan,ajay
Const in Objects
Example: It describes that the object properties can be modified only reference to the object cannot be changed.
const person = {
first_name: "Pankaj",
last_name: "Singh",
Age: 20,
About: "Web Developer and Competitive Programmer"
};
console.log(person);
// It is possible
person.first_name = "Aryan";
person.last_name = "Yadav";
person.Age = 22;
person.About = "Commerce undergraduate";
console.log(person);
// it is not possible
// const person={
// "first_name":"Aryan",
// "last_name":"Yadav",
// "Age":22,
// "About":"Commerce undergraduate"
// }
Output
{ first_name: 'Pankaj', last_name: 'Singh', Age: 20, About: 'Web Developer and Competitive Programmer' } { first_name: 'Aryan', last_name: 'Yadav', Age: 22, About: 'Commerce undergradu...
Supported Browsers:
P.S: To clear your concept of var, const, and let please go through How to declare variables in different ways in JavaScript?