Factorial of an Input Number in JavaScript

What Is a Factorial? Top ↑

The factorial of a non-negative integer n, written n!, is the product of all positive integers from 1 through n. For example, 5! = 1 × 2 × 3 × 4 × 5 = 120. By definition, 0! = 1.

Calculate a Factorial Top ↑

The original input → Submit → output facility is retained. This modern version uses BigInt so integer factorials remain exact instead of quickly exceeding the safe-integer range of normal JavaScript numbers.

Complete Working Source Top ↑

<!doctype html>
<html lang="en">
<head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1"><title>JavaScript Factorial Demo</title></head>
<body>
<label for="t1">Enter a non-negative integer</label>
<div class="col-sm-4 px-0"><input type="number" class="form-control" id="t1" min="0" max="500" step="1"></div>
<button type="button" class="btn btn-primary btn-sm mt-2" id="b1">Submit</button>
<div id="d1" class="alert alert-success mt-3" aria-live="polite">Output here</div>
<script>
const input = document.getElementById("t1");
const output = document.getElementById("d1");
const submitButton = document.getElementById("b1");

function showFactorial() {
  const value = input.value.trim();
  const number = Number(value);

  if (!Number.isInteger(number) || number < 0 || number > 500) {
    output.textContent = "Enter an integer from 0 to 500.";
    return;
  }

  let factorial = 1n;
  for (let i = 2n; i <= BigInt(number); i += 1n) {
    factorial *= i;
  }

  output.textContent = `Factorial of ${number} is: ${factorial}`;
}

submitButton.addEventListener("click", showFactorial);
</script>
</body>
</html>

Limits and Edge Cases Top ↑

Factorial is defined here for non-negative integers. The demo limits input to 500 to avoid creating excessively large output in the page. Negative integers and decimal values are rejected.

Next lower value of any decimal number → ← JavaScript Basic JavaScript variable →

← JavaScript Math Reference

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

About the author   Watch on YouTube

Connect: