38. Count and Say

Description:

The count-and-say sequence is the sequence of integers with the first five terms as following:

1.     1
2.     11
3.     21
4.     1211
5.     111221

1 is read off as "one 1" or 11.
11 is read off as "two 1s" or 21.
21 is read off as "one 2, then one 1" or 1211.

Given an integer n, generate the nth term of the count-and-say sequence.

Note: Each term of the sequence of integers will be represented as a string.

Example:

Input: 1
Output: "1"

Example:

Input: 4
Output: "1211"

My code:

/**
 * @param {number} n
 * @return {string}
 */
var countAndSay = function(n) {
    // 
    if(n == 1) {
        return '1';
    }
    let i = 1, formerStr = '1', curStr = '';
    while(i < n) {
        let j = 0, num = formerStr[0], count = 0;
        curStr = '';
        while(formerStr[j] != null) {
            if(formerStr[j] == num) {
                count++;
                j++;
            } 
            if(formerStr[j] != num) { // j++以后继续判断是否符合
                curStr += count + num;
                count = 0;
                num = formerStr[j];
            }
        }
        formerStr = curStr;
        i++;
    }
    return curStr;
};

Note: 一个n的循环，一个a个b的循环，中间用if(formerStr[j] != num)判断a个是否结束