“This is the fifth day of my participation in the First Challenge 2022. For details: First Challenge 2022.”

ES series of articles

  • ES6 variable constant (let const)
  • Deconstruction assignment
  • ES5 Array traversal mode
  • ES6 Array traversal mode
  • Extension of arrays

ES5 array traversal

  • The for loop

  • forEach()

  • map()

  • filter()

  • some()

  • every()

  • reduce()

  • for in

The for loop

let list= [1, 2, 3];
for (let i = 0; i < list.lenght; i++）{ 
    console.log(list[i]) 
}
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ForEach () : returns no value, just calls func forEach element

array.forEach(function(elem,index,array)){ }

  • Elem required, representing the current element
  • Index Indicates the current element index. This parameter is optional
  • Array Indicates the array object that the current element belongs to. This parameter is optional
  • Break and Cuntinue are not supported
  • Empty arrays are not checked
let arr= [1, 2, 3]; Array. forEach(function(elem,index,array)){if (arr [I] == 2){break} console.log(elem,index)} ForEach (): can't get out of the loopCopy the code

Map () : Returns a new Array, each element as the result of calling func

array.map(function(value)){ }

  • The map return value forms a new array
  • Do not change the original array value
  • Empty arrays are not checked
let arr= [1, 2, 3];
let result=arr.map(function(value){
    value+=1
    return value 
}) 
console.log(arr,result)
// [1 2 3]     [2 3 4] 
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Filter () : Returns an array of elements that meet func criteria

array.filter(function(value)){ }

  • Returns an array containing the existing elements that match the criteria. An empty array is returned if there are no elements, and a new array is formed if there are
  • Do not change the original array
  • Empty arrays are not checked
let arr= [1, 2, 3];
let result=arr.map(function(value){
    return value == 2
}) 
console.log(arr,result)
// [1 2 3]  [2]
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Some () : Returns Boolean to determine if any elements are eligible for func

array.some(function()){ }

  • Boolean if one element in the array meets the criteria, return true, and the rest of the elements will not be checked. If no element satisfies the condition, return false
  • It doesn’t change the original array
  • Empty arrays are not checked
let arr= [1, 2, 3];
let result=arr.some(function(value){
    return value==2 
})
console.log(arr,result)
//[1 2 3] true
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Every () : Returns Boolean to determine whether each element meets the func criteria

array.every(function()){ }

  • Returns Boolean that checks whether all elements meet the condition, true if they do, false otherwise
  • It doesn’t change the original array
  • Empty arrays are not checked
let arr= [1, 2, 3];
let result=arr.every(function(value){ 
    return value==2
}) 
console.log(arr,result)
//[1 2 3] false
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Reduce () : Receives a function as an accumulator

array.reduce(function(prev,cur,index,array)){}

  1. Prev. The initial value, or the return value at the end of the calculation
  2. Cur. The current element
  3. The index. The index of the current element
  4. Array. The array object to which the current element belongs
  • The reduce() method takes a function as an accumulator, and each value in the array (from left to right) starts to shrink and eventually evaluates to a value
  • Reduce () does not perform callbacks on empty arrays.

Scenario 1: Sum over Reduce

let arr= [1, 2, 3];
let sum=arr.reduce(function(prev,cur,index,array){
    return prev + cur 
},0) 
console.log(sum)
// 6
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Scenario 2: Obtain the maximum value of reduce

let arr= [1, 2, 3];
let max=arr.reduce(function(prev,cur){
return Math.max(prev,cur)
})
console.log(max)
// 4
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Scenario 3: Reduce deduplication

let arr= [1, 2, 3, 2, 4]; let res=arr.reduce(function(prev,cur){ prev.indexOf(cur)==-1 && prev.push(cur) return prev },[]) console.log(res) // [1 2, 3]Copy the code

for in

  • Iterate over the index of a number group
  • The property methods of the stereotype are also iterated over when iterating through arrays
Array.prototype.foo=function(){ 
    console.log('foo') 
} 
for(let index in arr){    
    console.log(index,arr[index]) 
}
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A front end small white, if the article has the wrong content, welcome big guy to give directions to discuss!