gistfile1.js

function encode(str) {
    if (!str) { return; }
    if (/[^a-z]/i.test(str)) { throw Error('Letters only'); }

    var len = str.length,
        i   = 1,
        output = '',
        counter = 1;

    for (; i <= len; i++) {

        // Am I a repeat character?
        if (str[i] === str[i-1]) {
            // Yes
            counter++;
        } else {
            // No
            output += counter + str[i-1];
            counter = 1;
        }
    }

    return output;
}

console.log( encode('aaabbbccczz') );
// => 3a3b3c2z