local backAndForthGenerator = function (ease)
    return function(t, tMax, start, delta)
        if t / tMax < 0.5 then
            return ease(t, tMax * 0.5, start, delta)
        else
            return ease(tMax - t, tMax * 0.5, start, delta)
        end
    end
end
 
display.setStatusBar(display.statusBarHidden)
local square = display.newRect( 0, 0, 50, 50 )
local _W, _H = display.contentWidth, display.contentHeight

square.x = _W * 0.5
square.y = _H * 0.8
 
transition.to(square, {time = 1000, y = 30, transition = backAndForthGenerator(easing.outExpo), iterations = 10})