原生textarea模拟移动端微信输入框

1、默认时是1行

2、输入时超过当前行自动换行，窗口最多显示4行

3、使用textarea

CSS

<style type="text/css">
    #textarea {
        display: block;
        margin: 0 auto;
        overflow: hidden;
        width: 300px;
        font-size: 14px;
        height: 18px;
        line-height: 24px;
        padding: 2px;
    }

    textarea {
        outline: 0 none;
        border-color: rgba(82, 168, 236, 0.8);
        box-shadow: inset 0 1px 3px rgba(0, 0, 0, 0.1), 0 0 8px rgba(82, 168, 236, 0.6);
    }
</style>

HTML

<textarea id="textarea" placeholder="回复内容"></textarea>

JS

<script>
    /**
     * 文本框根据输入内容自适应高度
     * @param                {HTMLElement}        输入框元素
     * @param                {Number}                设置光标与输入框保持的距离(默认0)
     * @param                {Number}                设置最大高度(可选)
     */
    var autoTextarea = function(elem, extra, maxHeight) {
        extra = extra || 0;
        var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,
                isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),
                addEvent = function(type, callback) {
                    elem.addEventListener ?
                            elem.addEventListener(type, callback, false) :
                            elem.attachEvent('on' + type, callback);
                },
                getStyle = elem.currentStyle ? function(name) {
                    var val = elem.currentStyle[name];
                    if (name === 'height' && val.search(/px/i) !== 1) {
                        var rect = elem.getBoundingClientRect();
                        return rect.bottom - rect.top -
                                parseFloat(getStyle('paddingTop')) -
                                parseFloat(getStyle('paddingBottom')) + 'px';
                    };
                    return val;
                } : function(name) {
                    return getComputedStyle(elem, null)[name];
                },
                minHeight = parseFloat(getStyle('height'));
        elem.style.resize = 'none';
        var change = function() {
            var scrollTop, height,
                    padding = 0,
                    style = elem.style;
            if (elem._length === elem.value.length) return;
            elem._length = elem.value.length;
            if (!isFirefox && !isOpera) {
                padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'));
            };
            scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
            elem.style.height = minHeight + 'px';
            if (elem.scrollHeight > minHeight) {
                if (maxHeight && elem.scrollHeight > maxHeight) {
                    height = maxHeight - padding;
                    style.overflowY = 'auto';
                } else {
                    height = elem.scrollHeight - padding;
                    style.overflowY = 'hidden';
                };
                if(height > 96){
                    height = 96
                }
                style.height = height + extra - 3 + 'px';
                scrollTop += parseInt(style.height) - elem.currHeight;
                document.body.scrollTop = scrollTop;
                document.documentElement.scrollTop = scrollTop;
                elem.currHeight = parseInt(style.height);
            };
        };
        addEvent('propertychange', change);
        addEvent('input', change);
        addEvent('focus', change);
        change();
    };
    var text = document.getElementById("textarea");
    autoTextarea(text); // 调用
</script>

效果：