JavaScript: Add digits until there is only one digit

JavaScript Math: Exercise-99 with Solution

Sum Digits Until One Digit

Write a JavaScript program to add repeatedly all the digits of a given non-negative number until the result has only one digit.

For example:
Input: 39
Output: 3
Explanation: The formula is like: 3 + 9 = 12, 1 + 2 = 3.

Visual Presentation:

Test Data:
(47) -> 2
(9) -> 9

Sample Solution-1:

JavaScript Code:

/**
 * Function to add the digits of a number until there is only one digit left.
 * @param {number} n - The number to process.
 * @returns {number} - The result of adding digits until only one digit remains.
 */
function test(n) {
  if (isNaN(n) || n === 0) return 0;  // Return 0 if input is not a number or is 0
  total = n; // Initialize total with the input number
    while (n >= 10) {        // Loop until the number becomes a single digit
        total = 0; // Reset total
        while (n != 0) { // Loop to add digits of the number
            total += Math.floor(n % 10); // Add the last digit to total
            n = Math.floor(n / 10); // Remove the last digit
        }
        n = total; // Update n with the new total
    }
    
    return total; // Return the final total
};  
// Test cases
n = 49;
console.log("n = " +n);
console.log("Add digits of the said number until there is only one digit: "+test(n));

n = 9;
console.log("Original text: " +n);
console.log("Add digits of the said number until there is only one digit: "+test(n));

Output:

n = 49
Add digits of the said number until there is only one digit: 4
Original text: 9
Add digits of the said number until there is only one digit: 9

Flowchart:

Live Demo:

See the Pen javascript-math-exercise-98-1 by w3resource (@w3resource) on CodePen.

Sample Solution-2:

JavaScript Code:

/**
 * Function to add the digits of a number until there is only one digit left.
 * @param {number} n - The number to process.
 * @returns {number} - The result of adding digits until only one digit remains.
 */
function test(n) {
    if (isNaN(n) || n === 0) return 0; // Return 0 if input is not a number or is 0
    if (n < 10) return n; // Return the number itself if it is already a single digit
    return n % 9 === 0 ? 9 : n % 9; // If n is divisible by 9, return 9; otherwise, return the remainder of n divided by 9
};

// Test cases
n = 49;
console.log("n = " +n);
console.log("Add digits of the said number until there is only one digit: "+test(n));

n = 9;
console.log("Original text: " +n);
console.log("Add digits of the said number until there is only one digit: "+test(n));

Output:

n = 49
Add digits of the said number until there is only one digit: 4
Original text: 9
Add digits of the said number until there is only one digit: 9

Flowchart:

Live Demo:

See the Pen javascript-math-exercise-99-1 by w3resource (@w3resource) on CodePen.

For more Practice: Solve these Related Problems:

• Write a JavaScript function that repeatedly sums the digits of a number until a single digit remains, returning that digit.
• Write a JavaScript function that implements the digital root algorithm using a loop.
• Write a JavaScript function that uses recursion to reduce a number to its digital root.
• Write a JavaScript function that validates non-negative input before performing repeated digit summation.

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