javascript - 未捕获的类型错误 : cannot read property 'querySelectorAll' of null

Question

我正在尝试在网站上使用此移动菜单。 http://tympanus.net/codrops/2013/08/13/multi-level-push-menu/comment-page-8/#comment-466199

我有它的工作，但一个 ie11 用户报告错误，我在控制台中看到以下错误 未捕获的类型错误:无法读取 nullmlPushMenu._init 的属性“querySelectorAll”@mlpushmenu.js:89mlPushMenu @mlpushmenu.js:67(匿名函数)@(索引):1062

这是有问题的 js 文件的片段

function mlPushMenu( el, trigger, options ) {   
    this.el = el;
    this.trigger = trigger;
    this.options = extend( this.defaults, options );
    // support 3d transforms
    this.support = Modernizr.csstransforms3d;
    if( this.support ) {
        this._init();
    }
}

mlPushMenu.prototype = {
    defaults : {
        // overlap: there will be a gap between open levels
        // cover: the open levels will be on top of any previous open level
        type : 'overlap', // overlap || cover
        // space between each overlaped level
        levelSpacing : 40,
        // classname for the element (if any) that when clicked closes the current level
        backClass : 'mp-back'
    },
    _init : function() {
        // if menu is open or not
        this.open = false;
        // level depth
        this.level = 0;
        // the moving wrapper
        this.wrapper = document.getElementById( 'mp-pusher' );
        // the mp-level elements
        this.levels = Array.prototype.slice.call( this.el.querySelectorAll( 'div.mp-level' ) );
        // save the depth of each of these mp-level elements
        var self = this;
        this.levels.forEach( function( el, i ) { el.setAttribute( 'data-level', getLevelDepth( el, self.el.id, 'mp-level' ) ); } );
        // the menu items
        this.menuItems = Array.prototype.slice.call( this.el.querySelectorAll( 'li' ) );
        // if type == "cover" these will serve as hooks to move back to the previous level
        this.levelBack = Array.prototype.slice.call( this.el.querySelectorAll( '.' + this.options.backClass ) );
        // event type (if mobile use touch events)
        this.eventtype = mobilecheck() ? 'touchstart' : 'click';
        // add the class mp-overlap or mp-cover to the main element depending on options.type
        classie.add( this.el, 'mp-' + this.options.type );
        // initialize / bind the necessary events
        this._initEvents();
    },

特定的第 89 行在中间，所以在这里将它拉出来以供您定位

this.levels = Array.prototype.slice.call( this.el.querySelectorAll( 'div.mp-level' ) );

然后我在我的页脚中调用了插件的实例(那是索引行 1082

那个电话看起来像这样

<script>
    new mlPushMenu(
        document.getElementById( 'mp-menu' ),
        document.getElementById( 'trigger' ),
        { type : 'cover' }
    );
</script>

Answer 1

您的桌面站点没有 ID 为“mp-menu”的元素。当您调用 getElementById 方法时，您将得到 null 作为响应。然后将其分配给对象的 el 属性。当您尝试调用 querySelectorAll 时，您是在尝试从空值调用。

根据对上述问题的评论，mp-menu 元素仅出现在移动网站 上。如果是这种情况，此代码也应该仅在移动设备上加载。