ES2015 (ES6) introduced const keyword to define a new variable. The difference in const variable declaration than others is that it cannot be reassigned.
Properties:
- Cannot be reassigned.
- Block Scope
- It can be assign on the variable on declaration line.
- Primitive value.
- The property of a const object can be change but it cannot be change to reference to the new object
- The values inside the const array can be change, it can add new items to const arrays but it cannot reference to a new array.
- Re-declaring of a const variable inside different block scope is allowed.
- Cannot be Hoisted.
- Create only read only reference to value.
Example 1: It describes that the const variable cannot be reassigned.
<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.
<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.
<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.
<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 array cannot be change.
<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 object cannot be changed.
<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:
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