for of 不能遍历对象，for in 可以

对比两段对象的遍历代码

for in

let b = {m: 1,n: 2,p: 3,q: 4}

for (let i in b) {
  if (i == "p") {
    break
  }
  console.log(i,b[i]);
}

运行结果为：

m 1
n 2

for of

let b = {m: 1,n: 2,p: 3,q: 4}

for (let i of b) {
  if (i == "3") {
    break
  }
  console.log(i,b[i]);
}

运行则会报错

两者都可以遍历数组，for in 可以遍历出键和值，for of 只能遍历出值

对比两段数组的遍历代码

for in

let a = ['c', 'd', 'e', 'f']
for (let i in a) {
  if (i == 2) {
    break
  }
  console.log(i,a[i]);
}

运行结果为：

0 c
1 d

for of

let a = ['c', 'd', 'e', 'f']

for (let i of a) {
  if (i == "e") {
    break
  }
  console.log(i);
}

运行结果为

c
d