css 通过fixed定位制作导航条的问题

#nav {
    position: sticky;
    top: 0;
}
最完美的解決方案。
可惜目前瀏覽器支持還不太好。

所以附上一個 polyfill：

var Sticky = function() {
    var s = [], 
        support = (function testSupport() {
        var div = document.createElement("div");
        var st = ["sticky", "-webkit-sticky"];

        return st.some(function(x) {
            div.style.position = x;

            return div.style.position === x;
        });
    }());

    function Sticky(o) {
        var self = this;

        s.push(self);

        self[0] = o;

        var placeholder = document.createElement("div");
        self.placeholder = placeholder;
        placeholder.classList.add("placeholder");

        self.fixed = false;

        self.posit = posit;

        function posit() {
            var rect;

            if (self.fixed) {
                rect = placeholder.getBoundingClientRect();

                self.staticTop = rect.top + window.pageYOffset;
            } else {
                rect = o.getBoundingClientRect();

                self.staticTop = rect.top + window.pageYOffset;
            }
        }

        posit();
    }

    Sticky.prototype.stick = function() {
        if (support)
            return;

        var o = this[0], 
            top = this.staticTop;

        var placeholder = this.placeholder, fixed = this.fixed;

        console.log(top);

        if (top > window.pageYOffset && fixed) {
            placeholder.parentNode.removeChild(placeholder);

            o.classList.remove("sticky");

            fixed = false;
        } else if (top <= window.pageYOffset && !fixed) {
            o.parentNode.insertBefore(placeholder, o);

            o.classList.add("sticky");

            fixed = true;
        }

        this.fixed = fixed;
    };

    if (!support) {
        window.addEventListener("scroll", function() {
            s.forEach(function(x) { x.stick(); });
        });

        window.addEventListener("resize", function() {
            s.forEach(function(x) { x.posit(); });
            s.forEach(function(x) { x.stick(); });
        });
    } else {
        console.log("support");
    }
    return Sticky;
}();
使用方法：new Sticky(document.querySelector("#nav"));