JavaScript Const - How to use 'const' keyword in JavaScript
JavaScript Assignment Operators
- The const keyword was introduced in ES6 (2015).
- Variables defined with const cannot be Redeclared.
- Variables defined with const cannot be Reassigned.
- Variables defined with const have Block Scope.
Cannot be Reassigned
- A const variable cannot be reassigned.
Sample Code
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript const</h2>
<p id="demo"></p>
<script>
try {
const PI = 3.141592653589793;
PI = 3.14;
}
catch (err) {
document.getElementById("demo").innerHTML = err;
}
</script>
</body>
</html>
Output
JavaScript const
TypeError: Assignment to constant variable
Correct
- const PI = 3.14159265359;
Incorrect
- const PI;
- PI = 3.14159265359;
When to use JavaScript const ?
Always declare a variable with const when you know that the value should not be changed.
- Use const when you declare.
- A new Array
- A new Object
- A new Function
- A new RegExp
Constant Arrays
- You can change the elements of a constant array:
// You can create a constant array:
const cars = ["kaashiv", "infotech", "welcome"];
// You can change an element:
cars[0] = "wikitechy";
// You can add an element:
cars.push("arrays");
But you can NOT reassign the array:
const cars = ["kaashiv", "infotech", "welcome"];
cars = ["venkat", "praveen", "soumith"]; // ERROR