The count-and-say sequence is a sequence of digit strings defined by the recursive formula: countAndSay(1) = "1" countAndSay(n) is the run-length encoding of countAndSay(n - 1). Run-length encoding (RLE) is a string compression method that works by

countAndSay.js

/**
 * Generates the nth element of the count-and-say sequence.
 *
 * @param {number} n - The position in the sequence (1-based index).
 * @return {string} - The nth element of the count-and-say sequence.
 */
var countAndSay = function(n) {
    // Validate the input: must be between 1 and 30 (inclusive)
    if (n < 1 || n > 30 || n == null) {
        return 'ERROR'; // Return an error if input is invalid
    }
    
    // Initialize the first sequence in the count-and-say sequence
    let previous_sequence = '1';
    
    // Loop to generate the sequence for n iterations
    for (let i = 1; i < n; i++) {
        let current_num = previous_sequence.charAt(0); // Start with the first character
        let count = 1; // Initialize the count for the first character
        let current_sequence = ""; // Initialize the current sequence
        
        // Process the previous sequence to generate the next one
        for (let j = 1; j < previous_sequence.length; j++) {
            let compare_num = previous_sequence.charAt(j); // Get the next character
            
            // If the current character changes, append the count and character to the result
            if (compare_num !== current_num) {
                current_sequence += `${count}${current_num}`; // Append count and current character
                current_num = compare_num; // Update current character
                count = 1; // Reset count for the new character
            } else {
                count++; // Increment count for consecutive identical characters
            }
        }
        
        // Append the last group (count and character) after finishing the loop
        current_sequence += `${count}${current_num}`;
        
        // Update the sequence for the next iteration
        previous_sequence = current_sequence;
    }
    
    // Return the nth sequence
    return previous_sequence;
};