下面这一段函数是什么意思呀？

Question

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