SOURCE

function generateStr(str) {
    const reg = /(\d+)\[(\w+)\]/g;
    let res = str;
    while(reg.test(res)){
        res = res.replace(reg, (...params) => {
            // console.log(params)
            return params[2].repeat(Number(params[1]));
        })
    }
    return res;
};
console.log(generateStr("2[e2[d]]"))