Decode String

let decodeString = function(str) {

    if (str.indexOf("[") < 0) {
        return str;
    }

    let openingIndex = [];
    let newStr = "";
    for (let i = 0; i < str.length; i++) {
        if (str[i] === "[") {
            openingIndex.push(i);
            newStr += str[i];
        }
        else if (str[i] === "]") {
            let opening = openingIndex.pop();
            let allMultipliers = str.substring(0, opening).match(/\d+/g);
            let multiplier = allMultipliers[allMultipliers.length - 1];
            let data = str.substring(opening + 1, i);
            if (data.indexOf("[") < 0) {
                newStr = newStr.substring(0, newStr.lastIndexOf("[") - multiplier.length) + data.repeat(multiplier);
                console.log({opening, multiplier, data, newStr});
            }
            else {
                newStr += str[i];
            }
        }
        else {
            newStr += str[i];
        }
    }

    return decodeString(newStr);
};