字符串压缩

 function ys(str) {
            let before = str.length;
            if (before < 5000) {
                let obj = {};
                for (i = 0; i < str.length; i++) {
                    if (!obj[str[i]]) {
                        obj[str[i]] = 1;
                    }
                    else {
                        obj[str[i]] += 1;
                    }
                }
                let arr = Object.entries(obj);
                function flatten(arr) {
                    let result = [];
                    for (let i = 0, len = arr.length; i < len; i++) {
                        if (Array.isArray(arr[i])) {
                            result = result.concat(flatten(arr[i]))
                        } else {
                            result.push(arr[i])
                        }
                    }
                    return result;
                    console.log(result);
                }
                let after = flatten(arr).length;
                //    console.log(after);
                if (before < after) {
                    console.log(str)
                } else {
                    let afterstr = arr.join();
                    let stra = afterstr.replace(/,/g, "")
                    console.log(stra);
                }
            } else {
                alter("请输入有效字符");
            }
        }