The JavaScript const keyword is used to define a new variable. Use it when you do not want to change the value of that variable for the whole program.
const varName = value; A normal variable value can be changed but const variable values can’t be changed. You don’t have to use var or let keyword while using the const keyword.
JavaScript const example
A simple example code const variable can’t be reassigned.
<!DOCTYPE html>
<html>
<body>
<script>
const number = 100;
try {
number = 99;
} catch (err) {
console.log(err);
}
console.log(number);
</script>
</body>
</html>
Output:
You can change the elements of a constant array by an index value.
const cars = ["A", "B", "C"];
// You can change an element:
cars[0] = "X";
// You can add an element:
cars.push("Y");But you can NOT reassign the array:
<script>
const cars = ["A", "B", "C"];
cars = ["X", "Y", "Z"]; // Uncaught TypeError: invalid assignment to const ‘cars’
</script>Same with constant Objects, You can change the properties of a constant object but can’t reassign the object:
Where to use JavaScript const?
Answer: Anytime you can declare a variable with const unless you know that the value will change. You use it with:-
- A new Array
- A new Object
- A new Function
- A new RegExp
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
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