什么是函数节流？

var main = {

    changeScreen: {
        init: function() {
            var _this = this;
            _this.obtainScreen();
            _this.resizeScreen();
        },

        obtainScreen: function() {
            var screenWidth,
                ua = navigator.userAgent.toLocaleLowerCase(),
                v = $.browser.version,
                $resizeScreen = $( "#resizeScreen" );

            screenWidth = window.innerWidth ? window.innerWidth :
                document.documentElement.clientWidth;

            if ( ua.indexOf( "msie" ) > -1 && +v < 10 ) { 
                if( screenWidth >= 1540 ) {
                    $resizeScreen.addClass( "w1240" );
                } else {
                    $resizeScreen.removeClass( "w1240" );
                }
            }
        },

        //函数节流的原理挺简单的，估计大家都想到了，那就是定时器
        throttle: function( fn, delay ) {
            var timer = null;
            return function() {
                var context = this, args = arguments;
                clearTimeout( timer );
                timer = setTimeout(function() {
                    fn.apply( context, args );
                }, delay );
            };
        },

        resizeScreen: function() {
            var _this = giftCommon.changeScreen;

            window.onresize = _this.throttle( _this.obtainScreen, 100 );
        }
    }

};