for、for..in、for..of 浅学习（2/16）

for --对象、数组

let arr = [1, 2, 3, 4];
for (let i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}                       
//1,2,3,4

let arr = { a1: 1, a2: 2, a3: 3 };
for (let i = 0; i < 3; i++) {
  console.log(arr[`a${i + 1}`]);
}
//1 2 3

for..in --对象、数组

let arr = [1, 2, 3, 4];
for (let i in arr) {
  console.log(i);
}                       
//0 1 2 3

let arr = { a: 1, b: 2, c: 3 };
for (let i in arr) {
  console.log(i);
}
//a b c

for..of --数组

let arr = [1, 2, 3, 4];
for (let i of arr) {
  console.log(i);
}                       
//1 2 3 4

for..of直接循环对象会报错