数组api

2020-07-21 本文已影响0人 komallqh

常用方法（4321+221）

4
push（）返回数组的长度
pop（）返回回最后一个元素
shift（）返回第一个元素
unshift（）返回数组长度
3
splice（i，n，m）可以添加或删除；返回删除的元素
slice(start,end) 返回切出来的数组，原数组不受影响
concat() 返回连接后的新数组；
concat与push区别：如果添加一个数组元素，concat是把数组中每项元素拆分后添加，push是直接将数组整体添加
2
reverse（）返回反转后的数组
sort（）改变原数组

let arr = [2,10,6,1,4,22,3]
console.log(arr.sort())   // [1, 10, 2, 22, 3, 4, 6]
let arr1 = arr.sort((a, b) =>a - b)  
console.log(arr1)   // [1, 2, 3, 4, 6, 10, 22]
let arr2 = arr.sort((a, b) =>b-a)  
console.log(arr2)  // [22, 10, 6, 4, 3, 2, 1]
console.log(arr)    // [22, 10, 6, 4, 3, 2, 1]

1
join

arrayObject.join(separator)

2
reduce(callback,initivalValue)

const array1 = [1, 2, 3, 4];
const reducer = (accumulator, currentValue) => accumulator + currentValue;

// 1 + 2 + 3 + 4
console.log(array1.reduce(reducer));
// expected output: 10

// 5 + 1 + 2 + 3 + 4
console.log(array1.reduce(reducer, 5));
// expected output: 15

reduceRight(callback,initialValue)

let arr = [0,1,2,3,4]
let arr1 = arr.reduceRight((preValue, curValue) => 
    preValue + curValue
)
console.log(arr1)    // 10

2
arr.indexOf（查找的项，开始查找的索引）

var beasts = ['ant', 'bison', 'camel', 'duck', 'bison'];

console.log(beasts.indexOf('bison'));
// expected output: 1

// start from index 2
console.log(beasts.indexOf('bison', 2));
// expected output: 4

console.log(beasts.indexOf('giraffe'));
// expected output: -1

arr.lastIndexOf

var animals = ['Dodo', 'Tiger', 'Penguin', 'Dodo'];

console.log(animals.lastIndexOf('Dodo'));
// expected output: 3

console.log(animals.lastIndexOf('Tiger'));
// expected output: 1

1
arr.includes() 判断是否有给定的值

let arr = [1,2,3,4,5]
let arr1 = arr.includes(2)  
console.log(arr1)   // ture

其他方法（53222）

forEach(callback)
遍历数组，无返回值； callback参数：value--当前索引值；index--索引；array --原数组；不会修改原数组！不会修改原数组！不会修改原数组！

let arr = [1,2,3,4,5]
arr.forEach( (value,index,array)=>{
        console.log(`value:${value}    index:${index}     array:${array}`)
    })   
    //  value:1    index:0     array:1,2,3,4,5
    //  value:2    index:1     array:1,2,3,4,5
    //  value:3    index:2     array:1,2,3,4,5
    //  value:4    index:3     array:1,2,3,4,5
    //  value:5    index:4     array:1,2,3,4,5

let arr = [1,2,3,4,5]
arr.forEach( (value,index,array)=>{
        value = value * 2
        console.log(`value:${value}    index:${index}     array:${array}`)
    })   
    console.log(arr)
    // value:2    index:0     array:1,2,3,4,5
    // value:4    index:1     array:1,2,3,4,5
    // value:6    index:2     array:1,2,3,4,5
    // value:8    index:3     array:1,2,3,4,5
    // value:10   index:4     array:1,2,3,4,5
    // [1, 2, 3, 4, 5]

map(callback)
遍历数组，返回一个新数组；不改变原数组；

let arr = [1,2,3,4,5]
arr.map( (value,index,array)=>{
        value = value * 2
        console.log(`value:${value}    index:${index}     array:${array}`)
})   
console.log(arr)
var array1 = [1, 4, 9, 16];

// pass a function to map
const map1 = array1.map(x => x * 2);

console.log(map1);
// expected output: Array [2, 8, 18, 32]
console.log(array1);    //[1, 4, 9, 16]

arr.forEach()和arr.map()的区别
1. arr.forEach()是和for循环一样，是代替for。arr.map()是修改数组其中的数据，并返回新的数据。
2. arr.forEach() 没有return arr.map() 有return

filter（callback）

let arr = [1,2,3,4,5]
let arr1 = arr.filter( (value, index) => value < 3)
console.log(arr1)    // [1, 2]
var words = ['spray', 'limit', 'elite', 'exuberant', 'destruction', 'present'];

const result = words.filter(word => word.length > 6);

console.log(result);
// expected output: Array ["exuberant", "destruction", "present"]
console.log(words);     //['spray', 'limit', 'elite', 'exuberant', 'destruction', 'present']

every（callback）

数组元素全部满足条件，返回true
let arr = [1,2,3,4,5]
let arr1 = arr.every( (value, index) => value < 3)
console.log(arr1)    // false
let arr2 = arr.every( (value, index) => value < 10)
console.log(arr2)    // true
function isBelowThreshold(currentValue) {
  return currentValue < 40;
}

var array1 = [1, 30, 39, 29, 10, 13];

console.log(array1.every(isBelowThreshold));
// expected output: true

some（）

let arr = [1,2,3,4,5]
let arr1 = arr.some( (value, index) => value < 3)
console.log(arr1)    // true
let arr2 = arr.some( (value, index) => value > 10)
console.log(arr2)    // false
var array = [1, 2, 3, 4, 5];

var even = function(element) {
  // checks whether an element is even
  return element % 2 === 0;
};

console.log(array.some(even));
// expected output: true

arr.keys()

let arr = [1,2,3,4]
let arr2 = arr.keys()
for (let key of arr2) {
    console.log(key);   // 0,1,2,3
}

arr.values()

let arr = [1,2,3,4]
let arr1 = arr.values()
for (let val of arr1) {
     console.log(val);   // 1,2,3,4
}

arr.entries()

let arr = [1,2,3,4]
let arr1 = arr.entries()
for (let e of arr1) {
    console.log(e);   // [0,1] [1,2] [2,3] [3,4]
}

arr.findIndex(value,index,array)

let arr = [1,2,3,4,5,2,4]
let arr1 = arr.findIndex((value, index, array) =>value > 3)
console.log(arr1)   // 3

arr.find(value,index,array)

let arr = [1,2,3,4,5,2,4]
let arr1 = arr.find((value, index, array) =>value > 2)
console.log(arr1)   // 3

2
Array.from()------es6
伪数组转化为数组，只要有length就行

    let str = '12345'
console.log(Array.from(str))    // ["1", "2", "3", "4", "5"]
let obj = {0:'a',1:'b',length:2}
console.log(Array.from(obj))   // ["a", "b"]

Array.of()------es6
将一组值转化为数组，类似于声明数组

let str = '11'
console.log(Array.of(str))   // ['11']
//等价于
console.log(new Array('11'))  // ['11]


//但是new Array()有缺点，就是参数问题引起的重载
console.log(new Array(2))   //[empty × 2]  是个空数组
console.log(Array.of(2))    // [2]

2
arr.fill(target,start,end)
使用给定的值填充一个数组，填充完后会该变原来的数组, 会改变原数组；会改变原数组

    let arr = [1,2,3,4,5]
let arr1 = arr.fill(5)
console.log(arr1)  // [5, 5, 5, 5, 5]
console.log(arr)   // [5, 5, 5, 5, 5];                        
    let arr = [1,2,3,4,5]
let arr2 = arr.fill(5,2)
console.log(arr2)  //[1,2,5,5,5]
let arr3 = arr.fill(5,1,3)
console.log(arr3)  //[1,5,5,5,5]     因为改变了原数组，所以这里不是[1,5,5,4,5]

arr.copyWidthin()
参数：target（必选） start（可选） end（可选）；获取start到end间的元素替换掉数组中target开始对应的项; 会改变原来的数组！会改变原来的数组！会改变原来的数组！

let arr = [1,2,3,4,5,6,7]
let arr1 = arr.copyWithin(1)
console.log(arr1)   // [1, 1, 2, 3, 4, 5, 6]
console.log(arr)    // [1, 1, 2, 3, 4, 5, 6]
let arr2 = arr.copyWithin(1,2)
console.log(arr2)   // [1, 2, 3, 4, 5, 6, 6]
console.log(arr)    // [1, 2, 3, 4, 5, 6, 6]
let arr3 = arr.copyWithin(1,2,4)
console.log(arr3)   // [1, 3, 4, 4, 5, 6, 6]

不会修改原数组

    concat
    slice
    join
    split
    toString
    valueOf
    迭代方法

会修改原数组

    push
    pop
    shift
    unshift
    reverse
    sort
    splice

返回新数组

    concat
    arr.map(callback)

split
将字符串转化为数组
参考：https://www.cnblogs.com/sqh17/p/8529401.html

数组api

常用方法（4321+221）

其他方法（53222）

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