offsetTop of undefined

Question

我有以下脚本，当我在我的网站上时，我一直收到这样的错误：

Uncaught TypeError: Cannot read property 'offsetTop' of undefined

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// mouse-scroll

(function() {
    var delay = false;

    $(document).on('mousewheel DOMMouseScroll', function(event) {
        event.preventDefault();
        if(delay) return;

        delay = true;
        setTimeout(function(){delay = false},200)

        var wd = event.originalEvent.wheelDelta || -event.originalEvent.detail;

        var a= document.getElementsByTagName('section');
        if(wd < 0) {
            for(var i = 0 ; i < a.length ; i++) {
                var t = a[i].getClientRects()[0].top;
                if(t >= 40) break;
            }
        }
        else {
            for(var i = a.length-1 ; i >= 0 ; i--) {
                var t = a[i].getClientRects()[0].top;
                if(t < -20) break;
            }
        }
        $('html,body').animate({
            scrollTop: a[i].offsetTop
        });
    });
})();

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