SOURCE

let str="Rome was not built in a day";
let  map = []; 
for (let i = 0; i < str.length; i++) {
    let char = str[i];
    if (map[char]) {
        map[char]++; 
    } else { 
        map[char] = 1; 
    }
}
console.log("一共出现了"+Object.keys(map).length+"个字母")
for (let key in map) {
    console.log(key + "出现了" + map[key] + "次");
}