JavaScript Const
ES2015 (ES6) introduced the const keyword to define a new variable. The difference in const variable declaration to others is that it cannot be re-assigned.
Properties:
- Cannot be reassigned.
- It’s 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.
Syntax:
const const_name; const x;
Example 1: It describes that the const variable cannot be reassigned.
javascript
<script type="text/javascript"> const x = 12; x = 13; x += 1;</script> |
Output:
Uncaught TypeError: Assignment to constant variable.
Example 2: It describes the const variable which contains the Block Scope.
javascript
<script type="text/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);</script> |
Output:
90 77 45 22
Example 3: It describes the const variable and assigned it after declaration.
javascript
<script type="text/javascript"> const x; x = 12;</script> |
Output:
Uncaught SyntaxError: Missing initializer in const declaration
Example 4: It describes the const variable cannot be Hoisted.
javascript
<script type="text/javascript"> x = 3; console.log(x); const x;</script> |
Output:
Uncaught SyntaxError: Missing initializer in const declaration
Example 5: It describes that the array values can be modified only reference to the array cannot be changed.
javascript
<script type="text/javascript"> // 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());</script> |
Output:
pankaj, sumit, chandan, ajay pankaj, sumit, Narayan, ajay
Example 6: It describes that the object properties can be modified only reference to the object cannot be changed.
javascript
<script type="text/javascript"> 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" // }</script> |
Output:
{first_name:'Pankaj',last_name:'Singh',
Age: 20, About:'Web Developer and Competitive Programmer'}
About
:
"Commerce undergraduate"
Age
:
22
first_name
:
"Aryan"
last_name
:
"Yadav"
[[Prototype]]
:
Object
{first_name:'Aryan',last_name:'Yadav',
Age:22, About:'Commerce undergraduate'}
About
: "Commerce undergraduate"
Age
:22
first_name
:"Aryan"
last_name
:"Yadav"
[[Prototype]]
:ObjectSupported 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
P.S: To clear your concept of var, const, and let please go through How to declare variables in different ways in JavaScript?
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