// ⭐️ stringCountLetters()
// 'abcc' => { a: 1, b: 1, c: 2 }
// ⭐️⭐️ reduce() 比 for loop 快兩倍❗️❗️
function stringCountLetters(str) {
return str
.split('')
.reduce((dict, c) => (dict[c] = (dict[c] || 0) + 1, dict) , {});
}
// ⭐️ str.countLetters()
String.prototype.countLetters = function() {
return stringCountLetters(this);
};