Summary of practical methods for JS beginners to process arrays

myArr.join('-') // concatenate with '-' symbol

myArr.concat(arr1, arr2, ..., arrN)

myArr.sort() // Sort alphabeticallymyArr.sort(function(a, b) {
	return a - b
}) // In ascending order, descending order is b - a
// Arrow function writing myArr.sort((a, b) => a - b)

myArr.reverse()

myArr.push(item1, item2, ..., itemN)
myArr.unshift(item1, item2, ..., itemN)

myArr.shift()

myArr.pop() // delete the last element of the array myArr.pop(1) // delete the element with index 1 in the array

myArr.splice(index, count, item1, item2, ..., itemN)
// index required. Integer that specifies the position to add/remove items. Use negative numbers to specify the position from the end of the array. // count Required. The number of items to delete. If set to 0, items will not be removed // item1, item2, ..., itemN Optional. Adding new items to the array

myArr.forEach(function (item, index, arr) {
 if (index === 3) {
 item = 123
 } 
}) // Loop through the array and change the value of the element at index 3 to 123
// Arrow function writing myArr.forEach((v, i, arr) => if (i === 3) { v = 123 })

myArr.indexOf(item)

const newArr = myArr.slice(0, 1)
// Extract the elements of the array myArr from index 0 to 1 // Parameters:
// begin (optional): index value, accepts negative values, starts extracting elements from the original array from this index, the default value is 0.
// end (optional): index value (not included), negative values ​​are accepted, and the extraction of the original array elements ends before this index. The default value is the end of the array (including the last element)

const state = myArr.every(function (item, index, arr) {
 return item > 10
}) // Check if all elements of the array myArr are greater than 10 and return a Boolean value state
// Arrow function writing const state = myArr.every((v, i, arr) => v > 10)

const state = myArr.some(function (item, index, arr) {
 return item > 10
}) // Check if there are elements greater than 10 in the array myArr and return a Boolean value state
// Arrow function const state = myArr.some((v, i, arr) => v > 10)

const state = myArr.includes(3) // Check if element 3 exists in array myArr and return a Boolean value state
const state = myArr.includes(3, 3) // Check whether element 3 exists in array myArr starting from index 3 and return a Boolean value state

const newArr = myArr.filter(function (item, index, arr) {
 return item > 10
}) // Check if all elements in the array myArr are greater than 10 and return a new array newArr
// Arrow function writing const newArr = myArr.filter((v, i, arr) => v > 10)

const newArr = myArr.map(function (item, index, arr) {
 return item * 10
}) // Multiply all elements in the array myArr by 10 and return a new array newArr
// Arrow function writing const newArr = myArr.map((v, i, arr) => v * 10)

const newArr = myArr.map(function (item, index, arr) {
 return {
 id: item.id,
 newItem: '123'
 }
}) // Process the elements or new elements in the object element specified in the array myArr and return a new array newArr
// Arrow function writing const newArr = myArr.map((v, i, arr) => {
 return {
 id: v.id,
 newItem: '123'
 }
})

const val = myArr.find(function (item, index, arr) {
 return item > 10
}) // Returns the value val of the first element in the array myArr that is greater than 10, or undefined if there is no element

const val = myArr.findIndex(function (item, index, arr) {
 return item > 10
}) // Returns the index of the first element in the array myArr that is greater than 10, or -1 if there is none

const myArr = [1, 2, 3]
const sum = myArr.reduce(function(pre, cur, index, arr) {
 console.log(pre, cur)
 return pre + cur
})
console.log(sum)
// The output values ​​are // 1, 2
// 3, 3
// 6

const myArr = [1, 2, 3]
const sum = myArr.reduce(function(pre, cur, index, arr) {
 console.log(pre, cur)
 return prev + cur
}, 2)
console.log(sum)
// The output values ​​are // 2, 1
// 3, 2
// 5, 3
// 8

const myArr = [1, 2, 3, 4]
const result1 = myArr.reduce(function(pre, cur) {
 return pre + cur
})
const result2 = myArr.reduce(function(pre, cur) {
 return pre * cur
})
console.log(result1) // 6
console.log(result2) // 24

const myArr = ['liang','qi','qi','liang','ge','liang'] 
const arrResult = myArr.reduce((pre,cur) => {
 if (cur in pre) {
  pre[cur]++
 }else{
  pre[cur] = 1
 }
 return pre
},{})
console.log(arrResult) // Result: {liang: 3, qi: 2, ge: 1}

const myArr = [
 {
  name: 'liangqi',
  height: 55
 },{
  name: 'mingming',
  height: 66
 },{
  name: 'lele',
  height: 77
 }
]
const result = myArr.reduce((a,b) => {
 a = a + b.weigth
 return a
},0)
console.log(result) // Result: 198

Array.of() // []
Array.of(undefined) // [undefined]
Array.of(1) // [1]
Array.of(1, 2) // [1, 2]

const myArr = [1, [2, 3, [4, 5, [12, 3, "zs"], 7, [8, 9, [10, 11, [1, 2, [3, 4]]]]]]]
console.log(myArr.flat(Infinity)) // [1, 2, 3, 4, 5, 12, 3, "zs", 7, 8, 9, 10, 11, 1, 2, 3, 4]