js 数组循环的几种方法及返回值

for

简单的遍历操作，没有返回值

  let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9];
  for (let i = 0; i < arr.length - 1; i++) {
    console.log('i: ', i);   // 1, 2, 3, 4, 5, 6, 7, 8, 9
  }

forEach

简单的遍历操作，没有返回值

  let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9];
  let result = arr.forEach(item => {
   console.log('item: ', item);  // 1, 2, 3, 4, 5, 6, 7, 8, 9
  })
 console.log(result); //undefined

filter

对数组进行筛选，返回满足条件的数据

  let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9];
  let brr = arr.filter(item => {
   return item > 5;
  })
  console.log('brr: ', brr); // [6, 7, 8, 9]

every

对数组进行条件判断，返回布尔值 && （全部满足为true，有一个不满足则返回false）

  let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9];
  let brr = arr.every(item => {
   return item > 0;
  })
  console.log('brr: ', brr); //true

map

对数组进行处理，返回处理后的数据

  let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9];
  let brr = arr.map(item => {
   return item + 5;
  })
  console.log('brr: ', brr); // [6, 7, 8, 9, 10, 11, 12, 13, 14]

some

判断数组元素，只要有一个满足条件就返回true

     let arr = [1, 2, 3, 4, 5];
     let result = arr.some((item) => {
       return item > 3;
     })
     console.log(result); //   true;