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Objects in JavaScript Array, like arrays in other programming languages, are collections of data. In JavaScript, the data inside an array can be of different types and have methods for performing common operations on arrays.
declare array
There are three different ways of declaring
1. Conventional method
const hobbys = new Array()
hobbys[0] = 'Basketball'
hobbys[1] = 'Badminton'
hobbys[2] = 'swimming'
console.log(hobbys)
// [ 'Basketball', 'Badminton', 'swimming' ]
2. The concise way
const hobbys = new Array('Basketball', 'Badminton','swimming')
console.log(hobbys)
// [ 'Basketball', 'Badminton', 'swimming' ]
3. Literally
const hobbys = ['Basketball','Badminton','swimming']
console.log(hobbys)
// [ 'Basketball', 'Badminton', 'swimming' ]
Array object methods
1. forEach
The forEach() method is used to call each element of the array and pass the element to the callback function. There is no return value, which is essentially equivalent to a for loop, which executes the function function for each item. The original array will not be changed.
// currentValue:必需,当前元素 index:可选,当前元素的索引值 arr:可选,当前元素所属的数组对象。
array.forEach(function(currentValue, index, arr))
let array = ['a', 'b', 'c']
let func = (currentValue, index, arr) => {
currentValue += 's'
console.log('currentValue：' + currentValue + ' index：' + index + ' arr：' + arr)
}
array.forEach(func)
console.log(array)
// 控制台输出：
// currentValue：as index：0 arr：a,b,c
// currentValue：bs index：1 arr：a,b,c
// currentValue：cs index：2 arr：a,b,c
// [ 'a', 'b', 'c' ]
2. map
Processes each element of the array with the specified function and returns the processed array.
The map() method returns a new array whose elements are the processed values of the original array elements after calling the function. The method processes elements sequentially in the original array element order. The original array will not be changed.
// currentValue:必须,当前元素的值 index:可选,当前元素的索引值 arr:可选,当前元素属于的数组对象
array.map(function(currentValue,index,arr))
let array = [1, 2, 3, 4, 5]
let result = array.map((item) => {
return item += 5
})
console.log(array)
console.log(result)
// [ 1, 2, 3, 4, 5 ]
// [ 6, 7, 8, 9, 10 ]
3. concat
Methods in JavaScript concat() are used to concatenate two or more arrays and return the result.
// array1, array2, ..., arrayN 必需,该参数可以是具体的值,也可以是数组对象,可以是任意多个
array1.concat(array2,array3,...,arrayN)
const array1 = ['a', 'b', 'c']
const array2 = ['d', 'e', 'f']
const array3 = array1.concat(array2)
console.log(array3)
const array4 = array1.concat('123')
console.log(array4)
// [ 'a', 'b', 'c', 'd', 'e', 'f' ]
// [ 'a', 'b', 'c', '123' ]
4. push
Methods in Javascript arrays push() are used to add one or more elements to the end of the array and return the new length.
let fruits = ["Banana", "Orange", "Apple", "Mango"]
let length = fruits.push("Kiwi")
console.log(fruits)
console.log(length)
// [ 'Banana', 'Orange', 'Apple', 'Mango', 'Kiwi' ]
// 5
5. unshift
The unshift() method adds one or more elements to the beginning of the array and returns the new length.
let fruits = ["Banana", "Orange", "Apple", "Mango"]
let length = fruits.unshift("Lemon", "Pineapple")
console.log(fruits)
console.log(length)
// [ 'Lemon', 'Pineapple', 'Banana', 'Orange', 'Apple', 'Mango' ]
// 6
6. pop
The pop() method is used to remove the last element of the array and return the removed element.
let sites = ['Google', 'Runoob', 'Taobao', 'Zhihu', 'Baidu']
let result = sites.pop()
console.log(sites)
console.log(result)
// [ 'Google', 'Runoob', 'Taobao', 'Zhihu' ]
// Baidu
7. shift
The shift() method is used to remove the first element of the array from it and return the value of the first element
let fruits = ["Banana", "Orange", "Apple", "Mango"];
let result = fruits.shift()
console.log(fruits)
console.log(result)
// [ 'Orange', 'Apple', 'Mango' ]
// Banana
8. splice
The splice() method is used to add or delete elements in the array and return an array of deleted elements
// 参数 Values: index: 必需,规定从何处添加/删除元素
// howmany: 可选,规定应该删除多少元素 必须是数字,但可以是 "0"
// item1, ..., itemX 可选,要添加到数组的新元素
array.splice(index,howmany,item1,.....,itemX)
let fruits = ["Banana", "Orange", "Apple", "Mango"]
let result = fruits.splice(1, 2, "Lemon", "Kiwi")
console.log(fruits)
console.log(result)
// [ 'Banana', 'Lemon', 'Kiwi', 'Mango' ]
// [ 'Orange', 'Apple' ]
9. slice
The slice() method returns selected elements from an existing array. It is also possible to extract a part of a string and return the extracted part as a new string. The original array will not be changed.
// start: 可选,规定从何处开始选取 若为负值,表示从原数组中的倒数第几个元素开始提取
// end: 可选,规定从何处结束选取 如果没有指定该参数,那么切分的数组包含从start到数组结束的所有元素
array.slice(start, end)
let fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"]
let result1 = fruits.slice(1, 3)
let result2 = fruits.slice(2)
console.log(fruits)
console.log(result1)
console.log(result2)
// [ 'Banana', 'Orange', 'Lemon', 'Apple', 'Mango' ]
// [ 'Orange', 'Lemon' ]
// [ 'Lemon', 'Apple', 'Mango' ]
10. join
The join() method combines all array elements into a string. It behaves like toString(), but you can also specify the delimiter
// separator: 可选,指定要使用的分隔符 如果省略该参数,则使用逗号作为分隔符
array.join(separator)
let fruits = ["Banana", "Orange", "Apple", "Mango"];
let energy1 = fruits.join();
let energy2 = fruits.join('-');
console.log(energy1)
console.log(energy2)
// Banana,Orange,Apple,Mango
// Banana-Orange-Apple-Mango
11. every
The every() method is used to check whether all elements of the array meet the specified condition (provided by the function).
array.every(function(currentValue,index,arr))
let ages = [32, 33, 16, 40]
let nums = [32, 33, 19, 40]
function checkAdult(age) {
return age >= 18
}
function checkNums(num) {
return num >= 18
}
// 16不满足大于18，故结果false
let result1 = ages.every(checkAdult)
// 每一项都满足条件，故结果true
let result2 = nums.every(checkNums)
console.log(result1)
console.log(result2)
// false
// true
12. filter
The filter() method creates a new array whose elements are checked by checking all elements in the specified array that meet the criteria. The original array will not be changed.
array.filter(function(currentValue,index,arr), thisValue)
let ages = [32, 33, 16, 40];
function checkAdult(age) {
return age >= 18;
}
let result = ages.filter(checkAdult)
console.log(result)
// [ 32, 33, 40 ]
13. indexOf
The indexOf() method returns the position within the string of the first occurrence of a specified string value. Returns -1 if not found
// searchvalue: 必需。规定需检索的字符串值。
// start: 可选的整数参数。规定在字符串中开始检索的位置。值：0~array.length-1
string.indexOf(searchvalue,start)
let str = "Hello world, welcome to the universe.";
// 输出w所在的下标索引13（空格也算），没有找到会返回-1
let n = str.indexOf("welcome");
console.log(n)
console.log(str[n])
// 13
// w
14. reduce
The reduce() method receives a function as an accumulator, and each value in the array (from left to right) is initially reduced and finally computed to a value.
array.reduce(function(total, currentValue, currentIndex, arr), initialValue)
let numbers = [2, 3, 5, 6]
function getSum(total, num) {
return total + num
}
let result = numbers.reduce(getSum, 0)
console.log(result)
// 16
15. reverse
The reverse() method is used to reverse the order of the elements in the array. will change the original array and return the array with the order changed.
let fruits = ["Banana", "Orange", "Apple", "Mango"]
let resut = fruits.reverse()
console.log(fruits)
console.log(resut)
// [ 'Mango', 'Apple', 'Orange', 'Banana' ]
// [ 'Mango', 'Apple', 'Orange', 'Banana' ]
16. sort
The sort() method is used to sort the elements of an array. The sort order can be alphanumeric or ascending or descending.
// sortfunction: 可选。规定排序顺序。必须是函数。
array.sort(sortfunction)
let fruits = ["Banana", "Orange", "Apple", "Mango"]
let ages = [9, 3, 4, 5, 7, 10]
// 升序
let agesFunAsc = function (ag1,ag2) {
return ag1 - ag2
}
// 降序
let agesFunDes= function (ag1,ag2) {
return -(ag1 - ag2)
}
fruits.sort()
ages.sort(agesFunAsc)
console.log(fruits)
console.log(ages)
ages.sort(agesFunDes)
console.log(ages)
// [ 'Apple', 'Banana', 'Mango', 'Orange' ]
// [ 3, 4, 5, 7, 9, 10 ]
// [ 10, 9, 7, 5, 4, 3 ]
17. toString
The toString() method is used to convert a number to a string.
number.toString(radix)
let num = 15
let n = num.toString()
// 也可以使用不同的进制把一个数字转换为字符串
// 2进制
let b = num.toString(2);
// 8进制
let c = num.toString(8);
// 16进制
let d = num.toString(16);
console.log(n)
console.log(b)
console.log(c)
console.log(d)
// 15
// 1111
// 17
// f
18. at
at()The method accepts an integer value and returns the value at the index, both positive and negative. A negative integer means counting down from the last item in the array.
array.at(index)
let str = 'helso word'
let item1 = str.at(2)
let item2 = str.at(-1)
console.log(item1)
console.log(item2)
// l
// d
19. find
The find() method returns the value of the first element of the array that passes the test (judged within the function).
array.find(function(currentValue, index, arr),thisValue)
let ages = [3, 10, 18, 20];
function checkAdult(age) {
return age >= 18;
}
let value = ages.find(checkAdult)
console.log(value)
// 18
20. some
The some() method is used to check whether the elements in the array meet the specified condition (provided by the function).
array.some(function(currentValue,index,arr),thisValue)
let ages = [3, 10, 19, 20];
function checkAdult(age) {
return age > 18;
}
let result = ages.some(checkAdult)
console.log(result)
// true
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