[方法]判断某个字母在字符串中出现的次数

部分场景下还是能用到的，比如游戏

var str = 'To be, or not to be, that is the question.';
var count = 0;
var pos = str.indexOf('e');
while (pos !== -1) {
count++;
pos = str.indexOf('e', pos + 1);
}
console.log(count) //4