A utility function to fill gaps in arrays

Here's a neat little helper function that allows you to fill any gaps in arrays. I've mostly used it when fetching analytics from a database and I don't want to bother filling in the gaps using SQL.

It can probably be improved a lot, but this is at least a working version.

/**
 * Safeguard to prevent an infinite loop (optional but recommended)
 */
export const MAX_GAP_ITERATIONS = 1000;

export const fill = <T>(
  arr: T[],
  compare: (prev: T, current: T) => boolean,
  fill: (prev: T) => T
) => {
  const filledArr: T[] = [];

  for (const item of arr) {
    // skip first item so we can compare to previous
    if (filledArr.length === 0) continue;

    let n = 1;

    // add items to array until there isn't a gap any more
    while (compare(filledArr[filledArr.length - 1], item)) {
      filledArr.push(fill(filledArr[filledArr.length - 1]));
      n++;

      if (n >= MAX_GAP_ITERATIONS) {
        throw new Error("error: gap is too big");
      }
    }

    filledArr.push(item);
  }

  return filledArr;
};

Example usage

// array with gaps
const arr = [1,2,6,8]

const filledArr = fill(
  arr,
  // there's a gap if the diff between previous and current is over 1
  (prev, curr) => curr - prev > 1,
  // new value should be 1 larger than previous
  (prev) => prev + 1
)

// filledArr === [1,2,3,4,5,6,7,8]