You can define a const variable using the const keyword in JavaScript. In the same way, you declare variables with let and var.
const var_name= value;The value of a constant can’t be changed through reassignment (i.e. by using the assignment operator), have Block Scope and it can’t be redeclared.
Note: JavaScript const variables must be assigned a value when they are declared:
JavaScript const variable
Simple example code variable cannot be reassigned.
<!DOCTYPE html>
<html>
<body>
<script>
const PI = 300;
try{
PI = 3.14;
PI = PI + 10;
}catch(error){
console.log(error)
}
console.log(PI)
</script>
</body>
</html>
Output:
Const variable cannot be hoisted
<script>
x = 100;
document.write(x);
const x; //Syntax Error
</script> const variable cannot be initialized after declaration
<script>
const x;
x = 100; //Syntax Error
document.write(x);
</script> Block scope
if (MY_FAV === 7) {
// this is fine and creates a block scoped MY_FAV variable
// (works equally well with let to declare a block scoped non const variable)
let MY_FAV = 20;
// MY_FAV is now 20
console.log('my favorite number is ' + MY_FAV);
// this gets hoisted into the global context and throws an error
var MY_FAV = 20;
}
// MY_FAV is still 7
console.log('my favorite number is ' + MY_FAV);Do comment if you have any doubts or suggestions on this Js const topic.
Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.
OS: Windows 10
Code: HTML 5 Version