Как получить количество строк в < textarea > используя JavaScript?

Я нашел гораздо более простой способ сделать это, но вам нужно будет установить высоту строки текстового поля в CSS. Я попытался прочитать высоту строки внутри скрипта ta.style.lineHeight , но, похоже, он не возвращает значения.

CSS

#ta { width: 300px; line-height: 20px; }

HTML

<textarea id="ta">Lorem ipsum dolor sit amet, consectetur adipiscing elit. Quisque suscipit, nisl eget dapibus condimentum, ipsum felis condimentum nisi, eget luctus est tortor vitae nunc. Nam ornare dictum augue, non bibendum sapien pulvinar ut. Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia Curae; Cras congue congue purus, quis imperdiet tellus ornare in. Nulla facilisi. Nulla elementum posuere odio ut ultricies. Nullam tempus tincidunt elit eget posuere. Pellentesque sit amet tellus sapien. Praesent sed iaculis turpis. Nam quis nibh diam, sed mattis orci. Nullam ornare adipiscing congue. In est orci, consectetur in feugiat non, consequat vitae dui. Mauris varius dui a dolor convallis iaculis.</textarea>

Скрипт

 var taLineHeight = 20; // This should match the line-height in the CSS
 var taHeight = ta.scrollHeight; // Get the scroll height of the textarea
 ta.style.height = taHeight; // This line is optional, I included it so you can more easily count the lines in an expanded textarea
 var numberOfLines = Math.floor(taHeight/taLineHeight);
 alert( "there are " + numberOfLines + " lines in the text area");

Обновление: спасибо к @Pebbl для устранения ошибок, это код, необходимый для получения высоты текстового содержимого ( демонстрация )

var calculateContentHeight = function( ta, scanAmount ) {
    var origHeight = ta.style.height,
        height = ta.offsetHeight,
        scrollHeight = ta.scrollHeight,
        overflow = ta.style.overflow;
    /// only bother if the ta is bigger than content
    if ( height >= scrollHeight ) {
        /// check that our browser supports changing dimension
        /// calculations mid-way through a function call...
        ta.style.height = (height + scanAmount) + 'px';
        /// because the scrollbar can cause calculation problems
        ta.style.overflow = 'hidden';
        /// by checking that scrollHeight has updated
        if ( scrollHeight < ta.scrollHeight ) {
            /// now try and scan the ta's height downwards
            /// until scrollHeight becomes larger than height
            while (ta.offsetHeight >= ta.scrollHeight) {
                ta.style.height = (height -= scanAmount)+'px';
            }
            /// be more specific to get the exact height
            while (ta.offsetHeight < ta.scrollHeight) {
                ta.style.height = (height++)+'px';
            }
            /// reset the ta back to it's original height
            ta.style.height = origHeight;
            /// put the overflow back
            ta.style.overflow = overflow;
            return height;
        }
    } else {
        return scrollHeight;
    }
}

var calculateHeight = function() {
    var ta = document.getElementById("ta"),
        style = (window.getComputedStyle) ?
            window.getComputedStyle(ta) : ta.currentStyle,

        // This will get the line-height only if it is set in the css,
        // otherwise it's "normal"
        taLineHeight = parseInt(style.lineHeight, 10),
        // Get the scroll height of the textarea
        taHeight = calculateContentHeight(ta, taLineHeight),
        // calculate the number of lines
        numberOfLines = Math.ceil(taHeight / taLineHeight);

    document.getElementById("lines").innerHTML = "there are " +
        numberOfLines + " lines in the text area";
};

calculateHeight();
if (ta.addEventListener) {
    ta.addEventListener("mouseup", calculateHeight, false);
    ta.addEventListener("keyup", calculateHeight, false);
} else if (ta.attachEvent) { // IE
    ta.attachEvent("onmouseup", calculateHeight);
    ta.attachEvent("onkeyup", calculateHeight);
}

Вы можете получить доступ к полю через Javascript DOM и просто подсчитать количество символов новой строки.

oArea = document.getElementById('myTextField');
var aNewlines = oArea.value.split("\n");
var iNewlineCount = aNewlines.length();