ES2015 (ES6) introduced the const keyword to define a new variable. The basic difference between const variable declaration and other variables are that it cannot be reassigned.
Properties:
- Const cannot be reassigned.
- It’s Block Scope
- It can be assigned to the variable on the declaration line itself.
- 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.
- Const cannot be Hoisted.
- Const creates only read-only references to value.
Syntax:
const const_name; const x;
Example 1: It describes that the const variable cannot be reassigned again.
JavaScript
const x = 12; x = 13; x += 1; |
Output:
Uncaught Type Error: Assignment to constant variable.
Example 2: Following example describes that const variables contains the Block Scope.
JavaScript
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
Example 3: It describes the const variable and assigned value to it after declaration.
JavaScript
const x; x = 12; |
Output:
Uncaught SyntaxError: Missing initializer in const declaration
Example 4: Following example shows that const variable cannot be Hoisted.
JavaScript
x = 3; console.log(x); const x; |
Output:
Uncaught SyntaxError: Missing initializer in const declaration
Example 5: Following example describes that the array values can be modified only but reference to the array cannot be changed.
JavaScript
// Changing the content of array is // possible in cost array const arr1 = ["a", "b", "c", "d"]; console.log(arr1.toString()); arr1[2] = "z"; // possible console.log(arr1.toString()); |
Output:
a, b, c, d a, b, z, d
Example 6: It describes that the object properties can be modified only but reference to the object cannot be changed.
JavaScript
const person = { first_name: "Rohit", last_name: "Singh", Age: 20, About: "Web Developer and Programmer"}; console.log(person); // It is possible person.first_name = "Aryan"; person.last_name = "kumar"; person.Age = 22; person.About = "Commerce undergraduate"; console.log(person); // it is not possible // const person={ // "first_name":"Aryan", // "last_name":"kumar", // "Age":22, // "About":"Commerce undergraduate" // } |
Output:
{
first_name: 'Rohit',
last_name: 'Singh',
Age: 20,
About: 'Web Developer and Programmer'
}
{
first_name: 'Aryan',
last_name: 'kumar',
Age: 22,
About: 'Commerce undergraduate'
}
Supported Browsers:
- chrome 21 and above
- Edge 12 and above
- Firefox 36 and above
- Internet Explorer 11 and above
- Opera 9 and above
- Safari 5.1 and above
Note: To clear your concept of var, const, and let you need to go through how to declare variables in different ways in JavaScript? We will post its complete article soon.