JavaScript 语句之常用 for 循环详解

let arr = [1, 2, 3, 4, 5, 6];
let obj = { a: 1, b: 2, c: 3 };
let map = new Map([['a', 'a1'], ['b', 'b2'], ['c', 'c3']]);
let set = new Set(['a', 'b', 'c']);

for (let i = 0; i < arr.length; i++) { // i是下标（索引）
  console.log(i)
  console.log(arr[i])
}

let arr = [1, 2, 3, 4, 5, 6];
Array.prototype.a = "1"

for (let i in arr) {  // i是下标（索引）
  console.log(i)
  console.log(arr[i])
}

let arr = [1, 2, 3, 4, 5, 6];
Array.prototype.a = "1"

for (let i in arr) {  // i是下标（索引）
  if (arr.hasOwnProperty(i)) {
    console.log(i)
    console.log(arr[i])
  }
}

let obj = { a: 1, b: '2', c: 3 };
Object.prototype.d = 4

for (let key in obj) {  // key是键
  console.log(key)
  console.log(obj[key])
}

for (let key in obj) {  // key是键
  if (obj.hasOwnProperty(key)) {
    console.log(key)
    console.log(obj[key])
  }
}

for (let key of arr.keys()) {  // key是下标
  console.log(key)
}
for (let value of arr) {     // value是值
  console.log(value)
}
for (let value of arr.values()) { // value是值
  console.log(value)
}
for (let [key, value] of arr.entries()) { // key是下标 value是值
  console.log(key,value)
}

for (let [key, value] of Object.entries(obj)) { // key是下标 value是值
  console.log(key, value)
}

for (let key of set.keys()) {  
  console.log(key)
}
for (let value of set) {     
  console.log(value)
}
for (let value of set.values()) { 
  console.log(value)
}
for (let [key, value] of set.entries()) { 
  console.log(key, value)
}

for (let key of map.keys()) { 
  console.log(key)
}
for (let value of map) {     
  console.log(value)
}
for (let value of map.values()) { 
  console.log(value)
}
for (let [key, value] of map.entries()) { 
  console.log(key, value)
}

for (let key of arr.keys()) {  // key是下标
  if (key == 3) {
    return
  }
  console.log(key)
}
for (let key of arr.keys()) {  // key是下标
  if (key == 3) {
    break
  }
  console.log(key)
}
for (let key of arr.keys()) {  // key是下标
  if (key == 3) {
    continue
  }
  console.log(key)
}

arr.forEach((value, index) => {
  console.log(value, index)
})

set.forEach((value, key) => {
  console.log(value, key)
})

map.forEach((value, key) => {
  console.log(value, key)
})

Illegal continue statement: no surrounding iteration statement

Illegal break statement