code challenge to encode a string

stringEncode.js

/**
 * Risyandi - 2022
 * how to create string to encode :
 * first we have a data array as input
 * for the example : [a, a, b, b, b, c, c] 
 * and for the expected result should be appear like this 
 * for the expected result : a2b3c2
 */

function stringEncode(input) {
    let lengthInput = input.length;
    let result = [];
    let counter = 1;
    for (let index = 0; index < lengthInput; index++) {
        if (input[index] === input[index + 1]) {
            counter++;
        } else {
            let resultString = input[index] + "" + counter
            result.push(resultString);
            counter = 1;
        }
    }
    result = result.join("");
    return result;
}

let input = ["a", "a", "c", "b", "b", "b", "c", "c"];
stringEncode(input);

/**
 * result will appear like this : a2c1b3c2
*/