为什么if里调用方法的时候没加括号？

window.onscroll = function () {

    if (checkScrollSLide) {

    };

}

function checkScrollSLide () {

     var main = document.getElementById('main');

     var box = getByClass(main, 'box');

     var lastBoxHeight = box[box.length - 1].offsetTop + Math.floor(box[box.length - 1].offsetHeight / 2);

     var scrollTop = document.body.scrollTop || document.documentElement.scrollTop;

     var bowserHeight = document.body.clientHeight || document.documentElement.clientHeight;

     return (lastBoxHeight < scrollTop + bowserHeight) ? true : false;

}