我猜你想要什么，但也许是这样？

I am guessing as to what you want, but maybe this?

var images = document.getElementsByTagName("img"),
    wanted;

if (images.length) {
    wanted = images[0].src;
    if (wanted) {
        wanted = wanted.split("?");
        if (wanted.length >= 2 && wanted[1].indexOf("&") !== -1) {
            wanted = wanted[1].split("&")[0];
        }
    }
}

if (typeof wanted !== "string") {
    wanted = "";
}

console.log(wanted);

输出

key=f430a2c1 

在 jsfiddle