Finding tidy numbers - JavaScript

A tidy number is a number whose digits are in non-decreasing order. We are required to write a JavaScript function that takes in a number and checks whether its a tidy number or not.

For example:

489 is a tidy number
234557 is also a tidy number
34535 is not a tidy number

Understanding Tidy Numbers

In a tidy number, each digit should be less than or equal to the next digit when reading from left to right. For instance, in 234789, we have 2 ? 3 ? 4 ? 7 ? 8 ? 9, making it tidy.

Method 1: Using Recursion

This approach processes digits from right to left, comparing each digit with the previous one:

const num = 234789;

const isTidy = (num, last = 10) => {
    if(num){
        if(num % 10 > last){
            return false;
        };
        return isTidy(Math.floor(num / 10), (num % 10));
    };
    return true;
};

console.log(isTidy(num));
console.log(isTidy(34535));
console.log(isTidy(489));

true
false
true

Method 2: Using String Conversion

Convert the number to a string and compare adjacent characters:

const isTidyString = (num) => {
    const str = num.toString();
    for(let i = 0; i  str[i + 1]){
            return false;
        }
    }
    return true;
};

console.log(isTidyString(234789));
console.log(isTidyString(34535));
console.log(isTidyString(111223));

true
false
true

Method 3: Using Array and every()

Convert to digit array and use the every() method for validation:

const isTidyArray = (num) => {
    const digits = num.toString().split('').map(Number);
    return digits.every((digit, index) => 
        index === 0 || digit >= digits[index - 1]
    );
};

console.log(isTidyArray(234789));
console.log(isTidyArray(987654));
console.log(isTidyArray(112233));

true
false
true

Comparison

Where d is the number of digits

Conclusion

All three methods effectively check for tidy numbers. The string conversion approach offers the best balance of readability and performance, while the array method provides a more functional programming style.