javascript reduce() 方法应用 —— 统计数组中每个元素出现的次数

let arr = ['abc','abd','abc','acd','abd','bcd','abc','abc','lmn','efj','lmn'];
let res = arr.reduce((prev,curr)=>{
	if(curr in prev){
		prev[curr]++;
	}else{
		prev[curr]=1;
	}
	return prev;
},{});
console.log(res);
console.log(arr);
运行结果：
{
abc: 4
abd: 2
acd: 1
bcd: 1
efj: 1
lmn: 2
}
["abc", "abd", "abc", "acd", "abd", "bcd", "abc", "abc", "lmn", "efj", "lmn"]