javascript - Javascript のデクリメント -- 私のスクリプトでは動作しません

Question

var backgroundImages = new Array(); // create an array holding the links of each image
        backgroundImages[0] = "style/images/bg0.png"; 
        backgroundImages[1] = "style/images/bg1.png";
        backgroundImages[2] = "style/images/bg2.png";
        backgroundImages[3] = "style/images/bg3.png";
        backgroundImages[4] = "style/images/bg4.png";
        backgroundImages[5] = "style/images/bg5.png";
        backgroundImages[6] = "style/images/bg6.png";

        var ImageCnt = 0;

        function nextImage(direction) // this should take into account the current value (starts at 3) and determines whether a higher or lower value should be returned based on the direction
        {
            if(direction == "left")
            {
            ImageCnt-- ;
            }
            if(direction == "right")
            {
            ImageCnt++ ;
            }
            document.getElementById("body-1").style.background = 'url(' + backgroundImages[ImageCnt] + ')'; //put's the new background together for rendering by using the returned value from nextImage()
            if(ImageCnt == 6)
            {
                ImageCnt = -1;
            }
        }

このスクリプトでは、ImageCnt ++ は関数 "nextImage('right')" では正常に動作していますが、ImageCnt-- をトリガーする nextImage('left') では関数が壊れます。私は何を間違っていますか？（jsの初心者）