js数组的13个题第一题：第二题第三题第四题第五题第六题第七题第八题第九题第十题第十一题第十二题第十三题

1
2
3
arr = [1,2,3,4]
item = 4
复制代码

1
2
3
4
5
6
7
8
9
10
11
12
13
let arr = [1,2,3,4]
let len = arr.length

let indexOf = (arr, item) => {
    for (let i = 0; i < len; i++){
        if(arr[i] === item){
            return i
        }
    }
    return -1
} 
console.log(indexOf(arr, 4))//3
复制代码

1
2
arr = [1,2,3,4]
复制代码

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
//方法一
let arr = [1,2,3,4]
function sum (arr){
    let num = 0
    arr.forEach (v => num += v )
    return num
}
console.log(sum(arr))//10

//方法二
let arr2 =[1,3,9,55,44,3,4]
let result = arr2.reduce((a,b) => a+b, 0)
console.log(result)//119

//a:  上次回调的结果
//b: 遍历的每一项数据
//0: a的初始值
复制代码

1
2
3
4
5
6
#输入
arr = [1, 2, 2, 3, 4, 2, 2]
item = 2
#输出
[1, 3, 4]
复制代码

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
//方法一
let arr = [1, 2, 2, 3, 4, 2, 2]
let item = 2
function filter(arr){
    let arr2 =[]
    arr.forEach(v => {
        if(v !== item){
            arr2.push(v)
        }
    })
    return arr2
}
console.log(filter(arr))//[1, 3, 4]

//方法二
let arr = [1, 2, 2, 3, 4, 2, 2]
let item = 2
let arr2 = arr.filter(v => v !== item)
console.log(arr2)//[1, 3, 4]
复制代码

1
2
3
4
5
6
7
#输入
item = 2
arr = [1, 2, 2, 3, 4, 2, 2]

#输出
[1, 3, 4]
复制代码

1
2
3
4
5
6
7
8
9
10
11
12
13
let arr = [1, 2, 2, 3, 4, 2, 2]
let item = 2
function fn(arr, item){
    //倒序遍历
    for(let i = arr.length - 1; i >= 0; i--){
        if(arr[i] === item){
            arr.splice(i, 1)
        }
    }
}
fn(arr, item)
console.log(arr)//[1, 3, 4]
复制代码

1
2
3
4
5
6
7
#输入
arr = [1,2,3,4]
item = 5
#输出
[1,2,3,4,5]

复制代码

1
2
3
4
5
6
7
8
9
10
11
12
let arr = [1,2,3,4]
let item = 5
function push(arr, item){
    let newArr = [...arr]
    newArr.push(item)
    //先复制数组 后对新数组push
    return newArr
}
console.log(arr)//[1, 2, 3, 4]
console.log(push(arr, item))// [1, 2, 3, 4, 5]

复制代码

1
2
3
4
5
6
#输入
arr = [1,2,3,4]
#输出
[1,2,3]

复制代码

1
2
3
4
5
6
7
8
let arr = [1,2,3,4]
function removeLast(arr){
    let newArr = arr.slice(0,arr.length - 1)
    return newArr
}
console.log(removeLast(arr))//[1,2,3]
console.log(arr)//[1,2,3,4]
复制代码

1
2
3
4
5
6
#输入
arr = [1, 2, 3, 4]
item = 5
#输出
[5, 1, 2, 3, 4]
复制代码

1
2
3
4
5
6
let arr = [1, 2, 3, 4]
let item = 5
let newArr = arr.slice()
newArr.unshift(item)
console.log(newArr)//[5, 1, 2, 3, 4]
复制代码

1
2
3
4
5
#输入
arr = [1, 2, 3, 4]
#输出
[2, 3, 4]
复制代码

1
2
3
4
5
6
7
let arr = [1, 2, 3, 4]
let newArr = []
for(let i = 1; i < arr.length; i++){
    newArr[i-1] = arr[i]
}
console.log(newArr)//[2, 3, 4]
复制代码

1
2
3
4
5
6
#输入
[1, 2, 3, 4], ['a', 'b', 'c', 1]
#输出
[1, 2, 3, 4, 'a', 'b', 'c', 1]

复制代码

1
2
3
4
5
6
7
8
9
10
11
12
//方法一
let arr1 = [1, 2, 3, 4]
let arr2 = ['a', 'b', 'c', 1]
let newArr = [...arr1, ...arr2]
console.log(newArr)//[1, 2, 3, 4, "a", "b", "c", 1]

//方法二
let arr1 = [1, 2, 3, 4]
let arr2 = ['a', 'b', 'c', 1]
let newArr = [].concat(arr1, arr2)
console.log(newArr)//[1, 2, 3, 4, "a", "b", "c", 1]
复制代码

1
2
3
4
5
6
7
8
##输入
arr = [1, 2, 3, 4]
item = 'z'
index = 2
#输出
[1, 2, 'z', 3, 4]

复制代码

1
2
3
4
5
6
7
8
let arr = [1, 2, 3, 4]
let item ="z"
let index = 2
    
let newArr = [...arr] //复制数组
newArr.splice(index, 0, item)
console.log(newArr)//[1, 2, "z", 3, 4]
复制代码

1
2
3
4
5
6
#输入
arr = [1, 2, 4, 4, 3, 4, 3]
item = 4
#输出
3
复制代码

1
2
3
4
5
6
let arr = [1, 2, 4, 4, 3, 4, 3]
let item = 4
let count = 0
arr.forEach(v => v === item && count++)
console.log(count)//3
复制代码

1
2
3
4
5
#输入
str = "123456789"
#输出
"987654321"
复制代码

1
2
3
4
5
let str = "123456789"
//先用split方法转成数组,再对数组调用reverse方法,再用join方法转成字符串
let str2 = str.split("").reverse().join("")
console.log(str2)//987654321
复制代码

1
2
3
4
5
6
function factorial(){
    // code
}
输出：
factorial(5) // 20
复制代码

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
//方法一
function factorial(n){
    // n = 5
    if(n === 1){
        return n
    }
     return n * factorial(n - 1) // 递归
}
console.log(factorial(5))

//方法二
function factorial(n, target = 1){
    if(n === 1)return target;
    return factorial(n - 1, n * target)//尾递归 
}
复制代码