var walk_the_DOM = function walk(node, func){
        func(node);
        node = node.firstChild;
        while (node) {
            walk(node,func);
            node = node.nextSibling;
        }
    };
    var getElementsByAttribute = function (att, value) {
        var results = [];
        walk_the_DOM(document.body,function (node) {
            var actual = node.nodeType === 1 && node.getAttribute(att);
            if (typeof actual === "string" && (actual === value || typeof value !== "string")) {
                results.push(node);
            }
        });
        return results;
    };

主要是if (typeof actual === "string" && (actual === value || typeof value !== "string"))这一句

圈住的那一部分