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How to create a constant array in JavaScript? Can we change its values? Explain.
To create a const array in JavaScript, we use the const keyword before the array name. While the array reference cannot be reassigned, individual elements can be modified.
A const array prevents reassignment of the entire array but allows mutation of its contents. This is because const creates an immutable binding, not an immutable value.
Syntax
const arrayName = [element1, element2, element3];
Example: Creating and Modifying a Const Array
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Const Array Example</title>
<style>
body {
font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
}
.result, .sample {
font-size: 18px;
font-weight: 500;
margin: 10px 0;
}
.result {
color: green;
}
button {
padding: 10px 15px;
background-color: #007bff;
color: white;
border: none;
cursor: pointer;
margin: 10px 0;
}
</style>
</head>
<body>
<h1>Const Array in JavaScript</h1>
<div class="sample"></div>
<div class="result"></div>
<button class="btn">CLICK HERE</button>
<h3>Click the button to modify array elements and attempt reassignment</h3>
<script>
let sampleEle = document.querySelector(".sample");
let resEle = document.querySelector(".result");
const arr = [22, 33, 44, 55];
sampleEle.innerHTML = "Original Array = [" + arr.join(", ") + "]";
document.querySelector(".btn").addEventListener("click", () => {
// Modifying individual elements (allowed)
arr[0] = 44;
arr[1] = 99;
arr[2] = 0;
resEle.innerHTML += "After modifying elements: [" + arr.join(", ") + "]<br>";
// Attempting to reassign the entire array (will throw error)
try {
arr = [11, 11, 11, 11];
} catch (err) {
resEle.innerHTML += "Error when reassigning: " + err.message;
}
});
</script>
</body>
</html>
Key Points
| Operation | Allowed? | Result |
|---|---|---|
Modify elements: arr[0] = 10
|
Yes | Elements change successfully |
Add elements: arr.push(6)
|
Yes | New elements added |
Reassign array: arr = [1, 2, 3]
|
No | TypeError: Assignment to constant variable |
Example: Array Methods with Const
<!DOCTYPE html>
<html>
<head>
<title>Const Array Methods</title>
</head>
<body>
<div id="output"></div>
<script>
const numbers = [1, 2, 3];
let output = document.getElementById("output");
output.innerHTML += "Original: [" + numbers.join(", ") + "]<br>";
// Adding elements
numbers.push(4, 5);
output.innerHTML += "After push: [" + numbers.join(", ") + "]<br>";
// Removing elements
numbers.pop();
output.innerHTML += "After pop: [" + numbers.join(", ") + "]<br>";
// Sorting
numbers.sort((a, b) => b - a);
output.innerHTML += "After sort: [" + numbers.join(", ") + "]<br>";
</script>
</body>
</html>
Making Arrays Truly Immutable
To create a completely immutable array, use Object.freeze():
<!DOCTYPE html>
<html>
<head>
<title>Frozen Array</title>
</head>
<body>
<div id="frozen-output"></div>
<script>
const frozenArr = Object.freeze([1, 2, 3]);
let output = document.getElementById("frozen-output");
output.innerHTML += "Original frozen array: [" + frozenArr.join(", ") + "]<br>";
// Attempt to modify (silently fails in non-strict mode)
frozenArr[0] = 99;
frozenArr.push(4);
output.innerHTML += "After modification attempts: [" + frozenArr.join(", ") + "]<br>";
output.innerHTML += "Array remains unchanged!";
</script>
</body>
</html>
Conclusion
Const arrays in JavaScript allow element modification but prevent reassignment. Use Object.freeze() for complete immutability when needed.
Updated on: 2026-03-15T23:18:59+05:30
2K+ Views
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