数组的扩展
数组的类 Array
数组的定义
var arr = [12,23]
var arr = new Array(3) 数字3代表 三个元素 或 3个空位
arr[0] = ""; arr[1] = "" ; arr[2] = ""
var arr = new Array("张"，"王")
var arr = new Array("1") //代表一个数组 且值为1
console.log(arr) //["1"]
var arr = new Array(1) //代表n个元素
console.log(arr) // [empty]
数组方法：
of方法 :
//of
var arr = Array.of(4)
console.log(arr) //[4]
返回一个元素为4的数组
foreach 方法:
//foreach() 遍历数组 返回值是undefined
var arr = ["王一博","肖战","魏婴","蓝忘机"]
arr.forEach((item,index) => {console.log(item)})
let arr = ["王一博","肖战","魏婴","蓝忘机"]
let as = arr.forEach((item,index) => {console.log(item)})
console.log(as) //undefined
map方法：
//map() 遍历数组 返回值是数组
map最大的功能将原来的数组 一一映射成一个新的数组,内部一定使用return
let arr = ["王一博","肖战","魏婴","蓝忘机"]
let as = arr.map((item,index) => {
return item;
})
console.log(as) // ["王一博","肖战","魏婴","蓝忘机"]
filter方法：
//filter() 过滤 从数组中找出所有符合条件的元素,并返回成一个新的数组,不改变原来的数组
let arr = [
{"name":"王一博","age":22},
{"name":"肖战","age":20},
{"name":"谭雅宁","age":19}
]
let as = arr.filter((item,index)=>{
//return 返回的值是true 就保留 返回的值是false 就不保留
return item.age>20;
})
console.log(as) // {name: "王一博", age: 22}
find方法：
//find() 对数组进行逐个查找,当返回值为true说明找到,直接返回结果,不再往下查找
let arr = [
{"name":"王一博","age":22},
{"name":"肖战","age":20},
{"name":"谭雅宁","age":19}
]
let as = arr.find((item,index) => {
return item.age>18;
})
console.log(as) //{name: "王一博", age: 22}
every() / some()
every():
//every() 判断数组中的每个元素是否满足条件 ,都满足条件 返回值是true 如果有一个不满足 就是 false
let arr = [
{"name":"王一博","age":22},
{"name":"肖战","age":20},
{"name":"谭雅宁","age":19}
]
let as = arr.every((item,index) => {
return item.age>20;
})
console.log(as) //false
some():
//some 判断数组中的元素是否满足条件,只要有一个满足就返回true , 如果都不满足就是false
let arr = [
{"name":"王一博","age":22},
{"name":"肖战","age":20},
{"name":"谭雅宁","age":19}
]
let as = arr.some((item,index) => {
return item.age>21;
})
console.log(as) //true
reduce():
//reduce() 迭代 从第二个元素执行,因为第一个元素没有上一个
//参数是一个回调函数 ,回调函数可以放三个参数 第一个prev 代表上一次reduce返回的值 第二个item是当前元素 第三个index 是当前索引
let arr = [1,2,3,4,5,6,7,8,9,10]
let as = arr.reduce((prev,item,index) => { //item 数组的元素从第二个开始 //prev 上一次的返回值 （第一个前面没有值)
console.log(prev) // 1 3 6 10 15 21 28 36 45
return prev+item;
})
console.log(as) //55
//如果想让reduce从第一个元素开始执行 (默认从第二个执行) 回调函数后面的参数就是第一次执行reduce()的prev
let arr = [1,2,3,4,5,6,7,8,9,10]
let as = arr.reduce((prev,item) => {
console.log(prev)
return prev+item;
},0) //代表从第一个元素开始,并且让他的返回值是0
console.log(as) //55
let name = ["tom","jim","jack","tom","jack"]
let as = name.reduce((prev,item) => {
if(item in prev){
prev[item]++
}else{
prev[item]=1
}
return prev;
},{})
console.log(as) // {tom: 2, jim: 1, jack: 2}