Finding greatest digit by recursion - JavaScript

We are required to write a JavaScript recursive function that takes in a number and returns the greatest digit in the number.

For example: If the number is ?

45654356

Then the return value should be 6

How It Works

The recursive approach extracts digits one by one from right to left using modulo (%) and integer division. Each digit is compared with the current maximum, and the function calls itself with the remaining digits.

Example

Following is the code ?

const num = 45654356;
const greatestDigit = (num = 0, greatest = 0) => {
   if(num){
      const max = Math.max(num % 10, greatest);
      return greatestDigit(Math.floor(num / 10), max);
   };
   return greatest;
};
console.log(greatestDigit(num));

Output

Following is the output in the console ?

6

Step-by-Step Breakdown

Let's trace through the execution with number 45654356:

const traceGreatestDigit = (num = 0, greatest = 0, step = 1) => {
   console.log(`Step ${step}: num=${num}, greatest=${greatest}`);
   if(num){
      const currentDigit = num % 10;
      const max = Math.max(currentDigit, greatest);
      console.log(`  Current digit: ${currentDigit}, New max: ${max}`);
      return traceGreatestDigit(Math.floor(num / 10), max, step + 1);
   };
   console.log(`Final result: ${greatest}`);
   return greatest;
};

traceGreatestDigit(45654356);

Step 1: num=45654356, greatest=0
  Current digit: 6, New max: 6
Step 2: num=4565435, greatest=6
  Current digit: 5, New max: 6
Step 3: num=456543, greatest=6
  Current digit: 3, New max: 6
Step 4: num=45654, greatest=6
  Current digit: 4, New max: 6
Step 5: num=4565, greatest=6
  Current digit: 5, New max: 6
Step 6: num=456, greatest=6
  Current digit: 6, New max: 6
Step 7: num=45, greatest=6
  Current digit: 5, New max: 6
Step 8: num=4, greatest=6
  Current digit: 4, New max: 6
Step 9: num=0, greatest=6
Final result: 6

Alternative Approach

Here's another recursive implementation that handles negative numbers:

const findGreatestDigit = (num) => {
   // Convert to positive and handle base case
   num = Math.abs(num);
   if (num 

6
9
7


Conclusion

Recursive digit extraction uses modulo and integer division to process each digit. The function maintains the maximum value found so far and returns it when no digits remain.