element attribute ID return undefined

问题

I tried to work with waypoints.js and make menu element active, when user scroll to it. As I can see, waypoints works well, but I can't get current section id... Console always said undefined.

So, what I'm doing wrong?

My script is:

jQuery(function() {
    var sections = jQuery('section');
    var navigation_links = jQuery('nav a');

    sections.waypoint({
    handler: function(direction) {
        var active_section;
        active_section = jQuery(this);
        if (direction === "up") active_section = active_section.prev();
        var active_link = jQuery('nav a[href="#' + active_section.attr("id") + '"]');
        navigation_links.parent().removeClass("active");
        active_link.parent().addClass("active");
        active_section.addClass("active-section");
        console.log(active_section.attr("id"))
    },
    offset: '35%'
    });
});

My HTML nav is:

<ul class="nav">
    <li><a href="#home">Home</a></li>
    <li><a href="#spec">What we do</a></li>
    <li><a href="#features">Features</a></li>
    <li><a href="#tools">Tools</a></li>
    <li><a href="#contacts">Contacts</a></li>
</ul>

My HTML section is:

<section id="home">
    <div class="wrap">
        <h1>...</h1>
    </div>
</section>

回答1:

In waypoints.js I've found that this refers to a waypoints internal object. If you console.log it though, you easily find how to select that element with jquery.

handler: function (direction){
    var DOMElement = $(this.element);
    console.log($(this.element).attr('data-id');
}

回答2:

As mentioned by @AmmarCSE the problem is indeed with if (direction === "up") active_section = active_section.prev();. Basically if the previous element does not have an id then undefined will be printed in the console. A way to avoid this is to check that the previous element has an id (check and make sure it's not undefined), an only then set active_section like so:

var prev_section_id = active_section.prev().attr("id");
if (direction === "up" && typeof prev_section_id != 'undefined') 
    active_section = active_section.prev();

Example Here

回答3:

This worked for me, only this will not return the id

$('.section').waypoint(function(){alert('this.element.id');});

来源：https://stackoverflow.com/questions/30816909/element-attribute-id-return-undefined

标签

javascript

jquery

jquery-waypoints