Who wants some sprinkles?? @El-Gregory does!!

sprinke.js

Array.prototype.sprinkle = function sprinkle(every, makeSprinkle) {
  let numSprinkles = 0
  return this.reduce((a, v, i) => {
    a.push(v)
    let doSprinkle = false
    if (typeof every === 'function') {
      doSprinkle = every(i, v, numSprinkles)
    } else {
      doSprinkle = i && i % every === 0
    }
    if (doSprinkle) {
      numSprinkles++
      const aSprinkle = (makeSprinkle(i, v, numSprinkles))
      if (aSprinkle) {
        a.push(aSprinkle)
      }
    }
    return a
  }, [])
}

use it like:

var arr = [1, 2, 3]
arr.sprinkle(i => i % 2, x => x * 2)

outputs:

[1, 2, 4, 3]