largeNumberFactorial.js

const factorial = (n) => {
    let res = [];

    // Initialize result
    res[0] = 1;
    let resSize = 1;

    // Apply simple factorial formula n! = 1 * 2 * 3 * 4...*n
    for (let x = 2; x <= n; x++) {
        resSize = multiply(x, res, resSize);
    }

    console.log("Factorial of given number is ");
    var s = "";
    for (let i = resSize - 1; i >= 0; i--) {
        s = s + res[i];
    }

    console.log(s);
}

// for multiplications
const multiply = (x, res, resSize) => {
    let carry = 0; // Initialize carry

    // One by one multiply n with individual digits of res[]
    for (let i = 0; i < resSize; i++) {
        let prod = res[i] * x + carry;
        res[i] = prod % 10; // Store last digit of 'prod' in res[]
        carry = Math.floor(prod / 10); // Put rest in carry
    }

    // Put carry in res and increase result size
    while (carry != 0) {
        res[resSize] = carry % 10;
        carry = Math.floor(carry / 10);
        resSize++;
    }

    return resSize;
}

factorial(25);
factorial(50);

/**
 * Sample output for above input:
 * Factorial of given number is
 * 15511210043330985984000000
 * Factorial of given number is
 * 30414093201713378043612608166064768844377641568960512000000000000
 *
 * Time complexity: O(log(N!))
 */