JavaScript数组：拼接和过滤器之间是否有交叉的方法？

Question

I have an array of objects, and want to:

1. Remove certain objects from the array
2. Treat the removed objects in a second step

I don't know in advance where these objects are. To recognize them, I need to use a function that queries their properties. It makes sense to retrieve the removed objects in a second array.

I had hoped to find a native method like filter or splice that would do this. Here's what I've come up with as a solution:

if (!Array.prototype.cherrypick) {
  Array.prototype.cherrypick = function(fn) {
    let basket = []
    let ii = this.length
    let item

    for ( ; ii-- ; ) {
      item = this[ii]

      if (fn(item)) {
        basket.unshift(item)
        this.splice(ii, 1)
      }
    }

    return basket
  }
}

Have I missed something? Is there a native method that does this already? Is my solution unsound in some way?

Answer 1

Methods such as Array.filter() returns a new array instead of changing the original array.

You can create a partition method using Array.reduce() that will return two arrays - those that passed the predicate, and those that failed:

 const partition = (predicate, arr) => arr.reduce((r, o) => { r[+!!predicate(o)].push(o); return r; }, [[], []]); const arr = [4, 8, 3, 10, 12]; const result = partition(n => n > 5, arr); console.log(result); 

And you can use the partition logic with Array.splice() to create the cherrypick method:

 if (!Array.prototype.cherrypick) { Array.prototype.cherrypick = function(predicate) { const [removedItems, items] = arr.reduce((r, o) => { r[+!!predicate(o)].push(o); return r; }, [[], []]); this.splice(0, arr.length, items); return removedItems; } } const arr = [4, 8, 3, 10, 12]; const removed = arr.cherrypick(n => n > 5); console.log('arr ', arr); console.log('removed ', removed); 

Answer 2

Have I missed something? Is there a native method that does this already?

No, most native utility methods try not to mutate the array and instead return a new one.

Is my solution unsound in some way?

Using splice and unshift repeatedly like you do is very inefficient. Better write

if (typeof Array.prototype.cherrypick == "function")
  console.warn("something already defines Array#cherrypick!");
Array.prototype.cherrypick = function(predicate) {
  let removed = [];
  for (let i=0, j=0; i<this.length; i++) {
    const item = this[i];
    if (fn(item)) {
      removed.push(item);
    } else {
      this[j++] = item; // keep in array, but at new position
    }
  }
  this.length = j; // removes rest
  return removed;
};

Answer 3

Just filter twice:

const picked = array.filter(fn);
array = array.filter((el, i, a) => !fn(el, i, a));

Answer 4

Use reduce as follows :

  array = [1,2,3,4,5,6,7];
  fn = n => n % 3 == 0;
  [array, picked] = array.reduce ( (r, el) => (r[+fn(el)].push (el), r), [[], []] ) 

Answer 5

Do you want something like this?

const basket = ['apple', 'banana', 'car'];
const filterMapBasket = basket
                       .filter(item => item !== 'car')
                       .map(item => {return { name: item }});

This will result the initial basket array of strings to be filtered and transformed to an array of objects.

Answer 6

This will alter the source array in place removing items meeting some test, and return those items...

 Array.prototype.removeIf = function(fn) { let i = this.length; let removed = []; while (i--) { if (fn(this[i], i)) { removed.push(...this.splice(i, 1)); } } return removed; }; let a = [0,1,2,3,4,5]; let removed = a.removeIf(i => i%2); console.log(a); console.log(removed);