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

function indexOf(arr, item) {
  //First determine whether the current browser supports indexOf method
  if (Array.prototype.indexOf){
      return arr.indexOf(item);
  } else {
      for (var i = 0; i < arr.length; i++){
          if (arr[i] === item){
              return i;
          }
      }
  }     
  return -1; //Always leak Return-1 outermost
}

2. Calculate the sum of all elements in a given array arr
2.1 Player Version

function sum(arr) {
    let res = 0;
    for(let i=0;i<arr.length;i++){
        res += arr[i];
    }
    return res;
}

2.2 version for Each

function sum(arr) {
    let res = 0;
    arr.forEach(function (curr,index,arr) {
        res += curr;
    });
    return res;
}
console.log(sum([1, 2, 3, 4, 5])); //15

Note: forEach() performs a callback function for each array element; unlike map() or reduce(), it always returns undefined values and is not chained.

function sum(arr) {
    let res = 0;
    arr.forEach(function (curr,index,arr) {
        return res += curr;
    });
}
console.log(sum([1, 2, 3, 4, 5])); //Return undefined

2.3 ES6 Player Edition

function sum(arr) {
    return arr.reduce((prev,curr) => prev + curr,0)
}
//Debugging part
console.log(sum([1, 2, 3, 4])); //10

3. Remove all elements in the array arr whose values are equal to item. Do not modify arr directly, the result returns a new array
3.1 Edition of Vegetable Chicken
If elements in an array are not equal to item s, add array elements to a new array

function remove(arr, item) {
    let res = [];
    for(let i=0;i<arr.length;i++){
        if(arr[i] !== item){
            res.push(arr[i]);
        }
    }
    return res;
}

Another Implementation of 3.2 Vegetable Chicken Version
If the elements in the array are equal to item, the code is no longer executed down.

function remove(arr, item) {
    let res = [];
    for(let i=0;i<arr.length;i++){
       if(arr[i] === item) continue;
       res.push(arr[i]);
    }
    return res;
}

3.3 slice & splice

• The meaning of the question requires not to modify arr directly, and the result returns a new array; slice() method can be used to intercept strings, and return the intercepted content (new array), we can pass parameter 0 to intercept all elements.
• You can't copy an array directly by assignment, which results in changing the elements of an array and the other array as well.
• To operate on a new array, splice() is used to cut the array, and splice() changes the original array.

function remove(arr, item) {
    let newArr = arr.slice(0); //
    for(let i=0;i<newArr.length;i++){
        if(newArr[i] === item){
            newArr.splice(i,1);
            i--;
        }
    }
    return newArr;
}

3.4 ES6 Player Edition (written by others)
The filter() method creates a new array in which the elements are checked for all qualified elements in the specified array. Returns an array of items that return true

function remove(arr,item){
    return arr.filter(ele => ele !== item);
}

4. Remove all elements of array arr whose values are equal to item, operate directly on a given array of arrs, and return the results.
4.1 If you operate directly on a given array, you can use the splice() method. Note that after the splice() method deletes the elements, the array length also changes, and you need to execute i - to avoid skipping the elements.

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

4.2 Delete backwards without considering the length of the array

function removeWithoutCopy(arr, item) {
    for(let i=arr.length-1;i>=0;i--){
        if(arr[i] === item){
            arr.splice(i,1);
        }
    }
    return arr;
}