总结JS数组用法

forEach和map

forEach会修改原来的数组，而map会得到一个新的数组并返回

let arr = [1, 2, 3, 4, 5];
arr.forEach((num, index) => {
  return (arr[index] = num * 2);
});
//arr = [2, 4, 6, 8, 10]

let arr = [1, 2, 3, 4, 5];
let doubled = arr.map(num => {
  return num * 2;
});
//doubled = [2, 4, 6, 8, 10]

findIndex, find, filter

let arr = [{ name: "swk" }, { name: "zwn" }, { name: "swj" }];
let index = arr.findIndex(item => item.name === "zwn");
//index = 1

let arr = [{ name: "swk" }, { name: "zwn" }, { name: "swj" }];
let obj = arr.find(item => item.name === "zwn");
//obj = {name: 'zwn'}

let arr = [{ name: "swk" }, { name: "zwn" }, { name: "swj" }];
let newArr = arr.filter(item => item.name != "zwn");
//newArr = [{name: 'swk'}, {name: 'swj'}]

sort

let yearArr = [2016, 2018, 2017, 2019];
yearArr.sort((a, b) => a - b);
//升序yearArr = [2016, 2017, 2018, 2019]
yearArr.sort((a, b) => b - a);
//降序yearArr = [2019, 2018, 2017, 2016]

reduce

//举个例子：按属性对object分类
let people = [
  { name: "Alice", age: 21 },
  { name: "Max", age: 20 },
  { name: "Jane", age: 20 },
];

function groupBy(objectArray, property) {
  return objectArray.reduce(function (acc, obj) {
    var key = obj[property];
    if (!acc[key]) {
      acc[key] = [];
    }
    acc[key].push(obj);
    return acc;
  }, {});
}
let groupedPeople = groupBy(people, "age");
// {
//   20: [
//     { name: 'Max', age: 20 },
//     { name: 'Jane', age: 20 }
//   ],
//   21: [{ name: 'Alice', age: 21 }]
// }

every

every测试一个数组内的所有元素是否都能通过某个指定函数的测试,它返回一个布尔值

[12, 5, 8, 130, 44].every(x => x >= 10);
//false

some

some测试数组中是不是至少有1个元素通过了被提供的函数测试,它返回的是一个Boolean类型的值

[12, 5, 8, 1, 4].some(x => x > 10);
//true

copyWithin

copyWithin方法浅复制数组的一部分到同一数组中的另一个位置，并返回它，不会改变原数组的长度

let numbers = [1, 2, 3, 4, 5];
numbers.copyWithin(-2, -3, -1);
// [1, 2, 3, 3, 4]