Get a random Y position for a new enemy being spawned, which is NOT "crowded" (no other active enemy must have the same rough Y position).

gistfile1.js

function getYPositionNotCrowded() {
  var maxCount = 1000
    , height   = 4
    , i = 0
    , y

  do {
    y = nextYPosition()
    if (++i > maxCount) { throw new Error("No position found!") }

  } while (isCrowded(y))

  return y


  //-- Helpers ----------------------------------
  function nextYPosition() {
    return Math.floor(Math.random()*WIDTH)
  }

  function isCrowded(y) {
    // Check if at least one enemy is in the way.
    // (In that case we know immediately that the y pos is crowded.)
    return enemies.some(function(enemy) {
      // Bounds
      var lower = enemy.y - height/2
        , upper = enemy.y + height/2

      // Check if within bounds
      return lower <= y && y <= upper
    })
  }
}