A Helper Method for Merging Associative Arrays in Ionic

This is the secret sauce. Let me know if you rewrite it more cleanly.

In Ionic, Infinite Scroll and Refresher load more content or reload it. With Infinite Scroll you often POST the current content ID and load only what follows; with Refresher you often clear existing items and reload. Depending on backend design—or when "some content was deleted, so I want to merge with what is already loaded"—you also get cases like this:

Like counts on a timeline may have changed, so I want to keep overwriting at least the latest 15 items
Some arrays may have been deleted, so I want to overwrite and reflect deletions
The update date changed, so I want to remove the old matching entry and bring in the latest

Merging associative arrays gets requirement-heavy once you think it through—"sort by ID, but for the same ID sort by created time ascending," and so on.
This has become something of a secret sauce for me, so I am publishing the associative-array merge method I built for my needs.

 public ArrayConcatById<T>(
    arrayOld: T[],
    arrayNew: T[],
    key: string,
    order: string = 'DESC',
    secondaryKey: string = null,
  ): T[] {
    if (!arrayNew.length && !arrayOld.length) {
      return [];
    }
    const lead = arrayNew[0][key] as number;
    const last = arrayNew[arrayNew.length - 1][key] as number;

    arrayOld = arrayOld || [];
    arrayNew = arrayNew || [];
    arrayOld = arrayOld.filter((vol) => {
      return (
        (lead > last && ((vol[key] as number) >= lead || (vol[key] as number) <= last)) ||
        (lead < last && ((vol[key] as number) <= lead || (vol[key] as number) >= last)) ||
        (lead as number) === (last as number)
      );
    });

    let old: T[];
    if (secondaryKey !== null) {
      old = arrayOld.filter((vol) => {
        let flg = true;
        arrayNew.forEach((element) => {
          if (element[secondaryKey] === vol[secondaryKey]) {
            flg = false;
          }
        });
        return flg;
      });
    } else {
      old = arrayOld;
    }

    const oldData = old.filter((vol) => {
      let flg = true;
      arrayNew.forEach((element) => {
        if (element[key] === vol[key]) {
          flg = false;
        }
      });
      return flg;
    });
    let data = arrayNew.concat(oldData);

    // Default is descending (DESC)
    let ord = -1;

    if (order === 'ASC') {
      // Ascending (ASC) when specified
      console.log('ASC');
      ord = 1;
    }

    data = data.sort((a, b) => {
      const x = a[key] as number;
      const y = b[key] as number;
      if (x > y) {
        return ord;
      }
      if (x < y) {
        return ord * -1;
      }
      return 0;
    });

    return data;
  }

Use it as a helper like this. The sort ID must be numeric.

this.items = this.helper.ArrayConcatById<Interface>(this.items, newItems, 'id', 'ASC');

If concat does not meet your array-merge needs, give it a try. If you refactor it into something cleaner, please tell me quietly. Actually, looking at it again after a long time, I want to rewrite this...

See you again.