js 点击下一张换图片

点击可以换图片

window.onload = function () {
var pren = document.getElementById('pren');
var cren = document.getElementById('cren');

var img = document.getElementsByTagName('img')[0];
var srt = ["../img/img1.jpg", "../img/img2.jpg", "../img/img3.jpg", "../img/img4.jpg",];
var index = 0;
pren.onclick = function () {
    index--;
    if (index < 0) {
        index = srt.length - 1;
    }
    img.src = srt[index]
};
cren.onclick = function () {
    index++;
    if (index > srt.length - 1) {
        index = 0;
    }
    img.src = srt[index]
};
}