Rearranging digits to form the greatest number using JavaScript

Problem

We are required to write a JavaScript function that takes in one positive three-digit integer and rearranges its digits to get the maximum possible number.

Approach

To create the maximum number, we need to arrange the digits in descending order. We can convert the number to a string, split it into individual digits, sort them in descending order, and join them back.

Example

Following is the code ?

const num = 149;
const maxRedigit = function(num) {
    if(num  999)
        return null
    return +num
    .toString()
    .split('')
    .sort((a, b) => b - a)
    .join('')
};
console.log(maxRedigit(num));

Output

941

How It Works

The function works by:

• First checking if the input is a valid three-digit number (100-999)
• Converting the number to a string using toString()
• Splitting the string into an array of individual digit characters
• Sorting the array in descending order using sort((a, b) => b - a)
• Joining the sorted digits back into a string and converting to number with +

Enhanced Example with Multiple Test Cases

const maxRedigit = function(num) {
    if(num  999) {
        return null;
    }
    return +num
    .toString()
    .split('')
    .sort((a, b) => b - a)
    .join('');
};

// Test with different numbers
console.log("Input: 149, Output:", maxRedigit(149));
console.log("Input: 532, Output:", maxRedigit(532));
console.log("Input: 777, Output:", maxRedigit(777));
console.log("Input: 204, Output:", maxRedigit(204));
console.log("Input: 99, Output:", maxRedigit(99));   // Invalid input

Input: 149, Output: 941
Input: 532, Output: 532
Input: 777, Output: 777
Input: 204, Output: 420
Input: 99, Output: null

Conclusion

This solution efficiently rearranges digits to form the maximum possible number by converting to string, sorting digits in descending order, and converting back to number. The function handles validation for three-digit numbers and returns null for invalid inputs.