textarea高度自增

		var observe;
    		if (window.attachEvent) {
		    observe = function (element, event, handler) {
		        element.attachEvent('on'+event, handler);
		    };
		}
		else {
		    observe = function (element, event, handler) {
		        element.addEventListener(event, handler, false);
		    };
		}
		function init () {
		    var text = document.getElementById('text');//textarea元素
		    function resize () {
		        text.style.height = 'auto';
		        text.style.height = text.scrollHeight+'px';
		    }
		   
		    function delayedResize () {
		        window.setTimeout(resize, 0);
		    }
		    observe(text, 'change',  resize);
		    observe(text, 'cut',     delayedResize);
		    observe(text, 'paste',   delayedResize);
		    observe(text, 'drop',    delayedResize);
		    observe(text, 'keydown', delayedResize);
		
		    text.focus();
		    text.select();
		    resize();
		}
		init()

通过获取textarea的scrollHeight来实现高度自增，类似微信的输入框。