遍历字符串

1. for 循环

let a = 'abcd';
for(let i = 0; i < a.length; i++) {
  console.log(a[i])
}

2. for…in…

let a = 'abcd';
for(let i in a){
  console.log(a[i])
}

3. for…of…

let a = 'abcd';
# for...of 循环能够直接以键名访问 item 的值
for(let i of a){
  console.log(i)
}