在javascript中搜索字母表中缺少的字母的字符串

function findwhatsMissing(s){
    var a = "abcdefghijklmnopqrstuvwxyz";
    //remove special characters
    s.replace(/[^a-zA-Z]/g,"");
    s = s.toLowerCase();
    //array to hold search results
    var hits = [];

    //loop through each letter in String
    for (var i = 0; i < a.length; i++) {
        var j = 0;
        //if no matches are found,push to array
        if (a[i] !== s[j]) {
                hits.push(a[i]);
        }
        else {
            j++;
        }
    }
    //log array to console
    console.log(hits);
}

但使用测试用例：
findwhatsMissing(“d a b c”);

结果将所有字母添加到缺少的数组之前.

任何帮助将不胜感激.