Simple upsert function

upsert.ts

/**
 * Updates or adds an item to a new array.
 * 
 * @param arr The array to check if item exists
 * @param item The item to add/insert 
 * @param comparer Run against each item in the array to see if it matches an existing item
 * @returns A new array with the item replaced if found, or added to the end if not found.
 */
export const upsert = <T,>(arr: T[], item: T, comparer: (existingItem: T, newItem: T, index: number) => boolean = (item1, item2) => item1 === item2): T[] => {
    const output = [];
    let found = false;

    for (let index = 0, end = arr.length; index < end; index++) {
        const existingItem = arr[index];
        const hasMatched = comparer(existingItem, item, index);

        if (hasMatched) {
            output.push(item);
            found = true;
        } else {
            output.push(existingItem);
        }
    }

    if (!found) output.push(item);

    return output;
};