Terse way to intersperse element between all elements in JavaScript array?

Question

Say I have an array var arr = [1, 2, 3], and I want to separate each element by an element eg. var sep = \"&\", so the output is [1, \"&a

Answer 1

export const intersperse = (array, insertSeparator) => {
  if (!isArray(array)) {
    throw new Error(`Wrong argument in intersperse function, expected array, got ${typeof array}`);
  }

  if (!isFunction(insertSeparator)) {
    throw new Error(`Wrong argument in intersperse function, expected function, got ${typeof insertSeparator}`);
  }

  return flatMap(
    array,
    (item, index) => index > 0 ? [insertSeparator(item, index), item] : [item] // eslint-disable-line no-confusing-arrow
  );
};