数组相关

1、数组搜索
1.1判断数组中是否存在某个值
①indexOf

indexOf.png

②lastIndexOf

lastIndexOf.png

③includes

includes.png

④find（返回符合条件的第一个元素,，无满足返回undefined）
⑤findIndex （返回符合条件的第一个元素的位置，无满足返回-1）

⑥filter（返回符合条件的所有元素的数组，无满足元素返回空数组）

1.2 map方法

map.png

//若arr为[],map返回的也是[]
let arr = [3000, 2000, 4500, 4200, 3500, 6000, 8000];
//const result = arr.map(item => {
//   return item * 2;
//})
const result = arr.map(item => item * 2)
console.log(result);//[6000,  4000, 9000,  8400, 7000, 12000,16000]

1.3reduce(累积操作)

image.png

//累加
let arr = [2, 4, 6, 8]

let result = arr.reduce((total, currentValue, currentIndex, arr) => {
    console.log(total);//10 12  16 22
    console.log(currentValue);//2,4,6,8
    console.log(currentIndex); //0,1,2,3
    console.log(arr); //[2, 4, 6, 8]
    return total + currentValue;
}, 10)
console.log(result);//30
//求平均值
let result = arr.reduce((total, currentValue, currentIndex, arr) => {
    total += currentValue;
    if (currentIndex == arr.length - 1) {
        return total / arr.length;
    }
    return total;
});

console.log(result);
//数组扁平化
//reduce
let arr = [
    1, [1, 2],
    [3, 4],
    [5, 6]
]
let result = arr.reduce((total, currentValue) => total.concat(currentValue), []);

console.log(result);
//递归
function flatten(arr) {
    let result = [];
    for (let i = 0; i < arr.length; i++) {
        if (Array.isArray(arr[i])) {
            result = result.concat(flatten(arr[i]));
        } else {
            result.push(arr[i]);
        }
    }
    return result;
}
console.log(flatten(arr));
//拓展运算符
unction flatten(arr) {
    while (arr.some(item => {
            Array.isArray(item)
        })) {
        return [].concat(...arr)
    }
}
console.log(flatten(arr));
console.log([].concat(...arr));
//toString + split
let result2 = arr.toString();
console.log(result2.split(','));
//flat
console.log(arr.flat());
//

1.4数组排序

//升序
let arr = ["Orange", "Apple", "Grapes", "Banana"];
let arr2 = [5, 40, 3, 9, 6];
console.log(arr.sort());//[ 'Apple', 'Banana', 'Grapes', 'Orange' ]
console.log(arr2.sort());//[ 3, 40, 5, 6, 9 ]
console.log(arr2.sort((a, b) => {//[ 3, 5, 6, 9, 40 
    return a - b;
}));
//字母降序
//先升序再 reverse
let arr = ["Orange", "Apple", "Grapes", "Banana"];
console.log(arr2.sort().reverse());
//如果数组中有undefined,undefined排在最后
//对象排序
//对象排序
const employess = [
    { eNo: 1001, salary: 3000 },
    { eNo: 1002, salary: 2200 },
    { eNo: 1003, salary: 3400 },
    { eNo: 1004, salary: 5000 },
]

employess.sort((a, b) => {
    return a.salary - b.salary
})
console.log(employess);
//[
  //{ eNo: 1002, salary: 2200 },
  //{ eNo: 1001, salary: 3000 },
  //{ eNo: 1003, salary: 3400 },
  //{ eNo: 1004, salary: 5000 } 
//]

1.5数组解构

let arr = [1,2,3,4];
let [a,b,c,d] = arr;
console.log(a,b,c,d);//1 2 3 4
//不完全结构
let arr = [1,2,3,4];
let [a,,c,d] = arr;
console.log(a,,c,d); // 1 3 4
//值交换
let a = 4;
let b = 5;
let [a,b] = let [b,a]
//与拓展运算符一起使用
let arr = [4, 5, 6];
let [a, ...b] = arr;
console.log(a);
console.log(b);