Summary of methods for operating arrays in js (code)

What this article brings to you is a summary (code) of methods for operating arrays in js. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

1. Find the position of the element item in the given array arr

function indexOf(arr, item) {
  if (Array.prototype.indexOf){ //判断原型中是否支持该方法
      return arr.indexOf(item);
  } else {      
  for (var i = 0; i < arr.length; i++){          
  if (arr[i] === item){              
  return i;
          }
      }
  }     
  return -1;
}

2. Calculate the sum of all elements in the given array arr

//forEach遍历：
function sum(arr) {
    var s = 0;
    arr.forEach(function(val, idx, arr) {
        s += val;
    }, 0);    
    return s;
};
//eval:
function sum(arr) {
    return eval(arr.join("+"));
};

3. Remove all elements in the array arr whose values ​​are equal to item. Do not modify the array arr directly. The result will be a new array

//splice()
function remove(arr,item){
    var newarr = arr.slice(0);    
    for(var i=0;i<newarr.length;i++){        
    if(newarr[i] == item){
            newarr.splice(i,1);
            i--;
        }
    }    
    return newarr;
}//push()
function remove(arr,item){
    var newarr = [];    
    for(var i=0;i<arr.length;i++){        
    if(arr[i] != item){
            newarr.push(arr[i]);
        }
    }    
    return newarr;
}//Array.prototype.filter()
function remove(arr,item){
    return arr.filter(function(ele){
         return ele != item;
    })
}

4. Remove all elements in the array arr whose values ​​are equal to item and operate directly on the given arr array. , and return the result

//splice
function removeWithoutCopy(arr, item) {
    for(var i=0; i<arr.length; i++)
    {        
    if(item == arr[i])
        {
            arr.splice(i,1);
            i--;
        }
    }    
    return arr;
}//while
function removeWithoutCopy(arr, item) {
    for(var i in arr){        
    while(arr[i]==item){
            arr.splice(i,1);
        }
    }    
    return arr;
}

5. Add the element item at the end of the array arr. Do not modify the array arr directly. The result will be a new array

//普通的迭代拷贝
var append = function(arr, item) {
    var length = arr.length,
        newArr = [];    
        for (var i = 0; i < length; i++) {
        newArr.push(arr[i]);
    }

    newArr.push(item);    
    return newArr;
};//使用slice浅拷贝+push组合
function append(arr, item) {
    var newArr=arr.slice(0);
    newArr.push(item);    
    return newArr;
}

6. Delete the last element of the array arr. Do not modify the array arr directly, the result will be a new array

//利用slice
function truncate(arr) {
    return arr.slice(0,-1);
}
//利用filter
function truncate(arr) {
    return arr.filter(function(v,i,ar) {
        return i!==ar.length-1;
    });
}
//利用push.apply+pop
function truncate(arr) {
    var newArr=[];
    [].push.apply(newArr, arr);
    newArr.pop();    return newArr;
}
//利用join+split+pop    注意！！！：数据类型会变成字符型
function truncate(arr) {
    var newArr = arr.join().split(',');
    newArr.pop();    
    return newArr;
}

7. Add the element item at the beginning of the array arr. Do not modify the array arr directly. The result will be a new array

//利用concat
function prepend(arr, item) {
    return [item].concat(arr);
}
//使用push.apply
function prepend(arr, item) {
    var newArr=[item];
    [].push.apply(newArr, arr);    
    return newArr;
}
//利用slice+unshift/splice
function prepend(arr, item) {
    var newArr=arr.slice(0);
    newArr.unshift(item);//newArr.splice(0,0,item);
    return newArr;
}

8. Delete the first element of the array arr. Do not modify the array arr directly, the result will be a new array

//利用slice
function curtail(arr) {
    return arr.slice(1);
}
//利用filter
function curtail(arr) {
    return arr.filter(function(v,i) {
        return i!==0;
    });
}
//利用push.apply+shift
function curtail(arr) {
    var newArr=[];
    [].push.apply(newArr, arr);
    newArr.shift();    
    return newArr;
}

9. Merge array arr1 and array arr2. Do not modify the array arr directly. The result will be a new array

//利用concat
function concat(arr1, arr2) {
    return arr1.concat(arr2);
}
//利用slice+push.apply
function concat(arr1, arr2) {
    var newArr=arr1.slice(0);
    [].push.apply(newArr, arr2);    
    return newArr;
}
//利用slice+push
function concat(arr1, arr2) {
    var newArr=arr1.slice(0);    
    for(var i=0;i<arr2.length;i++){
        newArr.push(arr2[i]);
    }    
    return newArr;
}

10. Add the element item at the index of the array arr. Do not modify the array arr directly, the result will be returned as a new array

//利用slice+splicefunction insert(arr, item, index) {
    var newArr=arr.slice(0);
    newArr.splice(index,0,item);    return newArr;
}//利用push.apply+splicefunction insert(arr, item, index) {
    var newArr=[];
    [].push.apply(newArr, arr);
    newArr.splice(index,0,item);    return newArr;
}

11. Count the number of occurrences of elements in the array arr whose value is equal to item

//filter()-->利用指定的函数确定是否在返回的数组中包含某一项
        function count(arr, item) {
            var count = arr.filter(function(a) {
                return a === item;   //返回true的项组成的数组
            });            
            return count.length;
        }  
        //map()-->对数组中的每一项进行给定函数，
  //返回每次函数条用的结果组成的数组；
        function count(arr, item) {
            var count = 0;
            arr.map(function(a) {
                if(a === item) {
                    count++;
                }
            });            
            return count;
        }//forEach()-->对数组中的每一项运行传入的函数
        function count(arr, item) {
            var count = 0;
            arr.forEach(function(a) {
                a === item ? count++ : 0;
            });            
            return count;
        }

12. Find the repeated elements in the array arr

//两次遍历
function duplicates(arr) {
     //声明两个数组，a数组用来存放结果，b数组用来存放arr中每个元素的个数
     var a = [],b = [];     
     //遍历arr，如果以arr中元素为下标的的b元素已存在，则该b元素加1，否则设置为1
     for(var i = 0; i < arr.length; i++){         
     if(!b[arr[i]]){
             b[arr[i]] = 1;             
             continue;
         }
         b[arr[i]]++;
     }     
     //遍历b数组，将其中元素值大于1的元素下标存入a数组中
     for(var i = 0; i < b.length; i++){         
     if(b[i] > 1){
             a.push(i);
         }
     }     
     return a;
 }
 //先排序，如果后一个与前一个相等且未保存，则保存。
 function duplicates(arr) {
    var a=arr.sort(),b=[];    
    for(var i in a){        
    if(a[i]==a[i-1] && b.indexOf(a[i])==-1) b.push(a[i]); 
    }    
    return b;
}
//先排序然后再判断function duplicates(arr) {
    var new_arr = arr.sort();//先把arr排序
    var res = [] ;//目标容器
    for( var i = 0 ; i < new_arr.length ; i++){        
    if(new_arr[i] == new_arr[i+1] &&
        new_arr[i] !=new_arr[i-1]){//判断是否重复,是否已经放入容器
            res.push(new_arr[i]);
        }
    }
    return res;

13. Find the second power of each element in the array arr. Do not modify the array arr directly, the result will be a new array

function square(arr) {
   //声明一个新的数组存放结果
     var a = [];
     arr.forEach(function(e){
         //将arr中的每一个元素求平方后，加入到a数组中
         a.push(e*e);
     });     
     return a;
 }
 // 使用map
  function square(arr) {
      return arr.map(function(e) {
          return e * e;
      })
  }  
  // ES6箭头函数版
  const square = arr => arr.map(e => e * e);

14. In the array arr, find all positions where the element whose value is equal to item appears

//forEach
function findAllOccurrences(arr, target) {
    var a=[];
    arr.forEach(function(e,index){
        if(e==target)
            a.push(index);
    })    
    return a;
}
function findAllOccurrences(arr, target) {
var temp = [];
    arr.forEach(function(val,index){
        val !== target ||  temp.push(index);
    });    
    return temp;
}//filter
function findAllOccurrences(arr, target) {
    var result=[];
    arr.filter(function(item,index){
        return item===target&&result.push(index);
    });    
    return result;
}

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