StringSequence.js

function getNextChar (char) {

    if (char == 'z') return 'A';
    if (char == 'Z') return 'a';
    
    return String.fromCharCode(char.charCodeAt(0) + 1);
}

function getNextString (str) {

    let last = str.slice(-1),
        init = str.slice(0, str.length - 1);

    if (last === 'Z') {
        let trail = 1, i = init.length - 1;
        for ( ; i > 0; --i) {
            if (init[i] === 'Z') {
                // Propagate the nexted char
                ++trail;
            } else {
                // We found a char that can be nexted
                break;
            }
        }
        str = init.slice(0, i)      +
              getNextChar(init[i])  +
              'a'.repeat(trail);
        return str;
    }
    return init + getNextChar(last);
}

function* sequenceString (str) {
    let newStr = getNextString(str);
    yield newStr;
    yield* sequenceString(newStr);
}

var k = sequenceString('ZaZZ');
for (var i = 0; i < 100; ++i) console.log(k.next())