Textarea Auto height

This using Pure JavaScript Code.

function auto_grow(element) {
    element.style.height = "5px";
    element.style.height = (element.scrollHeight)+"px";
}

textarea {
    resize: none;
    overflow: hidden;
    min-height: 50px;
    max-height: 100px;
}

<textarea oninput="auto_grow(this)"></textarea>

For those of us accomplishing this with Angular JS, I used a directive

HTML:

<textarea elastic ng-model="someProperty"></textarea>

JS:

.directive('elastic', [
    '$timeout',
    function($timeout) {
        return {
            restrict: 'A',
            link: function($scope, element) {
                $scope.initialHeight = $scope.initialHeight || element[0].style.height;
                var resize = function() {
                    element[0].style.height = $scope.initialHeight;
                    element[0].style.height = "" + element[0].scrollHeight + "px";
                };
                element.on("input change", resize);
                $timeout(resize, 0);
            }
        };
    }
]);

$timeout queues an event that will fire after the DOM loads, which is what's necessary to get the right scrollHeight (otherwise you'll get undefined)